How would the signals get to the SMD connector without vias? Some sort of blind vias? Are they waterproof and who says so? Anyway, to be honest this is such a bad idea from the start. Not just because of water, but because of salt, galvanization and probably a bunch of other problems too. Separate the PCB from the outside environment. You need to involve a good mechanical engineer with experience of maritime development. Salt water is really nasty business.

Lundin‭ Vias. I can tent the vias with solder mask. I would pick small via diameter (say 0.15-0.20 mm) which would mean that solder mask would plug the vias entirely. That would be the easiest solution, because it’s a bog-standard PCB fab process. If that’s not enough, the PCB fab can plug the vias. It would cost me extra, but it’s still a common PCB fab process. (No throughole components, obviously.)

Olin mentioned at the end of his answer that he’s doing an immersed capacitive level sensor, and it works. I bet there are vias, and they aren’t causing too many issues. (At the same time Olin’s situation is different, because he’s not trying to hold water with a PCB.)

Lundin‭ Corrosion. That’s an interesting question. The electrodes will be ENIG plated. If ENIG isn’t enough, I could do hard gold plating (like PCIe board edge connectors). The metals involved (from top to bottom) are gold, nickel, copper. Gold is a noble metal, so it shouldn’t react with seawater. Nickel doesn’t corrode in seawater. Copper reacts with water, so I need to seal copper against water. If solder mask overlaps the edges of the electrodes, then it protects the edges of the copper foil. Say the metal pad of the electrode is 5mm wide, and the solder mask opening is only 1mm wide. The only materials exposed to seawater would be silk screen (epoxy), and gold. These materials may be inert enough. That’s my hypothesis.

The slightest scratch will mean that the solder mask is leaking however. Solder mask is literally what it says: a means for preventing solder to end up on top layer traces. Using it for anything else (including isolation) is questionable practice. Anyway, the standard practice for anything in tough environments is: mould the whole thing. Or as a means to save a failed design, apply silicone all over the place.