I would say open world first. Note, for the first instance, it does not have to be a big open world (2-3 small islands, 2 small towns, 1 village)
I like the fact that you can already choose your ship and go into combat. That means basic game mechanics are in place.
Adding open world would close a game loop, and you would have a playable game, not instanced battles.
I would like to compare this development to Elite: Dangerous which launched today:
First they had Alphas - which was same as here. Ships, flight, graphics, and a little bit of combat (scenario based). Few ships just to test core mechanics. - To me it looks Naval Action is about to pass that stage Premium Betas - A Very limited Open world (not sure how many, but I think around 5 star systems, with 1-2 space stations) (out of a galaxy with millions of star systems). This gave them the chance to close the game loop, and see how would a person travel from A to B. Also how a player would interact with stations, and how a player would be getting in and out of combat. In my mind regarding Naval Action some stretch of sea, 2-3 smaller islands (i.e. 2 inhabited town, village whatever, and 1 sandbank style) should do it, and then expand from there Betas - up to 10 ships and 20 star systems in Elite (adding Trading, Exploring and Mining - 1 per beta1, beta2 etc). In terms of Naval Action (adding 1-2 more islands, perhaps 1 with bigger town protected by a fort), few more ships, and adding a exploration aspect, or trading Gammas - "Almost Done". With most of the mechanics in place, they were mostly balance, bug fixing, and adding content (more star systems, more ships), some game mechanics (Missions). In terms of Naval Action, hereby I would add a layer on top of all (ships going from town A to town B - why, what is their purpose - traders, raiders, etc)
Not to bore everyone too much, the core idea is VERY simple.
Whenever introducing NEW game mechanic with few examples (2-3 not more). I.e. 2 secret island locations to explore (or 2 ships for combat). This gives a chance to debug the mechanic, not the actual example. Then scale up from there
I am taking Elite Dangerous as an example, as their every release introduced something new, while refining and fine tuning other aspects. However I believe they are around 170 ppl strong which means for Naval Action this has to be taken into account.
My favorite game was Sea Dogs by Akella. No game after managed to get that same ship to ship combat feeling. AC4 did help popularize it though, however I agree they weighed more towards assasin gameplay than pirate lifestyle. Still it was a great game :-)
I have just pre-ordered Naval Action today, after seeing Sidestrafe play it. Can't wait for friday :-)