How it works currently:
1) Sail around until you find a sponge.
2) toggle on swimming and equip a diver's weight that takes both hands.
3) Carefully position your boat or ship near the sponge, but not too close that it blocks you from grabbing the sponge (since the game engine doesn't, at least currently, actually have the ability to consider you being "under" the boat.)
4) Exit the craft.
5) Right-click sponge from minimap like regular forageable.
6) Get back to the craft when you have the sponge.
Main problem with stages 4-6 is that you are risking permanent death to gain a relatively mediocre curio (even when dried on herbalist table.)
While risk of permanently dying is part of the game, in this case it's especially outrageous as your character's life is now entirely in the hands of factors beyond your control: powergrid, digital communication infrastructure between you and the physical server hardware, and the server itself. If any of these fail, game goes "no refunds" and lets your character do nothing as it drowns.
Outside of hacky solutions like "disconnected player in deep water tries to swim to any nearby boat/ship" I think best way is to significantly rework "dive into water foraging."
1) Have "diving kit" installed/equipped on the ship or rowboat.
2) Sail around until you find a "diving location."
3) Perform the dive as single action where server makes your character dive and return to the craft, only way character dies is if they get KO or so something/somebody destroys the boat/ship.
This system would probably still make people not dive sponges on characters they absolutely do not want to die, as Jorb seems to consider entering Deep Water as statement of "I consent to my character being easily killed."
(Diving kit made of molded lead weight, X amount of rope and Seaman's knife.)