I am mostly going to criticize but I am aiming to do so in a constructive way. I will split my points into major - regarding features, mechanics or lack of thereof and minor - mostly UI, usability and details.
Major:
1. The designer is terribly restrictive. In my opinion it is absolutely crucial to have more control over several design characteristics, the first of which would be hull shape. You have already stated that ships will get wider as well as longer with displacement and that's a good thing, but i also propose the following: add another slider underneath displacement slider called "hullform". That slider will control the length to width ratio while keeping set displacement. Hulls already have hullform statistic, which could be used as a middle point. Moving slider to one side would make the hull longer and thinner - improving the hullform statistic, acceleration, and centerline space. Moving the slider the other way would make the hull shorter and wider -> the armor belt and deck weight would decrease (because it is now shorter), there would be more space for side mounted guns and stability would improve (wider ship will lean over less in a turn).
2. Continuing with design characteristic that we need more control over - towers placement. As opposed to main battery turrets and funnels (which must be placed where the hull is deep enough) there is absolutely no reason why "this forward tower cannot be placed any further back, even though the hull is perfectly capable of supporting it". The only limitation to placing tower should be that rear tower must be aft of the forward tower. Even if you want to associate some sort of spotting penalties if the forward tower is in the rear half of the ship, the player should be able to build ships such as Nelsol and Rodnol Nelson and Rodney. There is already a weight balancing factor player needs to consider when placing things (and that's a good thing).
3. Casemates. Let us remove them. And not just leave them empty, add a button to the designer that says "Remove empty casemates", which will replace all casemates without guns with smooth hull. Having the empty/welded over casemates will be awesome if you're planning refitting ships in campaign (see visible spots were casemates were before removal on US Battleships), but having them on brand new ship designs is awkward.
4. Funnels and machinery. Machinery does provide a weight balancing aspect toward the middle of the ship and that's a good start, but I feel like it would be better (and more sensible) to tie the exact point where this weight is applied to the funnel placement. It should be underneath the funnel (or in case of multiple funnels, halfway between them) rather than the exact middle of the ship. It would open up a lot more balancing options, especially when trying to build ships like Nelson, where you could use the machinery to counterweight the all-forward armament.
5. OPTIONAL. As in "it would be cool and helpful to have in the game, but not strictly necessary". Allow the players to indirectly control the citadel size and thus its mass. The citadel would span from the most forward main battery turret/funnel to rearmost main battery turret/funnel. It would allow for additional incentive of all-forward armament (with shortening of the citadel and associated weight saving). You could also gate that ability (adjusting citadel size) with tech, to simulate the adoption of all-forward scheme.
6. Towers. Right now the towers are strictly on the basis of "this one is heavier, more expensive and better than the other one", I'm hoping more towers will be added and that there will be multiple in the same tier, for example: one that gives a bigger accuracy boost, one that gives a damage control boost, one that is better at spotting, etc. Make the player think "what do I want?" rather than "can i somehow fit the best tower on this hull". Also, I would hope for addition of towers with inbuilt barbettes also in a barbette-less versions (especially if said barbette on a capital ship tower is too small for capital ship-grade calibers)
7. I don't know if this is already a feature in the game, but I couldnt find any mention of it, so I'm mentioning it: There should be accuracy penalties if a lot of different caliber guns are shooting. (Difficulties with rangefinding and differentiating splashes from one another, especially if calibers are similar). It shouldnt matter much at short range (which is why pre-dreadnoughts had a very mixed gun armament), but become more relevant as range increases. If it's already in the game, congratulations, you deserve a cookie.
8. OPTIONAL. . There is a barbette armor slot on the left side of the screen. But what does it mean exactly? Will heavy armor on a barbette be the same regardless of whether ship has 17 or 5 inches of armor? Consider giving us an additional armor thickness box for barbette instead of that unspecified protection level. So that we can say "14 in turret armor, 12.5 in barbette".
9. Allow us to mount secondary guns on top of main battery turrets. It was often done on early dreadnoughts.
Minor:
1. Give as a button in the designer that will highlight all empty casemate mounts and display a number for their max size. It will save clicking through all casemates caliber trying to find ones that fit.
2. Let us use the forward underwater (?) torpedo launcher on the TB. The launcher is modelled as part of the hull, but you cant put anything in there, and i don't think its functional.
3. Restrict certain combinations of engine-fuel-boiler setup. Otherwise I present to you the brand new diesel engine fueled by coal and using induced draft boilers. (In that case probably combine all the bonuses into the diesel engine and completely disable the other two selection boxes)
4. Rotating side guns only rotates one of them, while the mirrored gun is pointing the default way.
5. Moving "incorrectly placed" mirrored guns only moves one of them
There might be more of the minor category but right now that's all that comes to my mind.
In summary, the designer feels good when designing pre-dreadnoughts, but when attempting to design a legitimate dreadnought it feels clunky and restrictive.There is however a lot of potential and space for improvements and i hoping there will be improvements, as this is still early access. I hope my criticism was helpful and if you need me to elaborate on any of the points above feel free to @ me.
Cheers