Tonguez
A suffusion of yellow
Nice necro!
I'm always in favor of a saving throw for the victim to attempt to mitigate the effect in some way.
Part Water does mention the trench fills back in slowly over a round, so it could be ruled it parts slowly as well. Vessels not completely in the area of effect might get a save (Water Vessel skill check?) to maneuver to safety (remember, things on a grid are static, in "real" life they're in constant motion). If the vessel is over 50% over the area, the save might be at disadvantage. If you want it to do damage, I'd have (seperate) saves (Dex) for the crew to ride out the fall, and maybe up to 8d4 falling damage on a fail. Success on the crew save indicates they've somehow managed to ride the descent safely. It'll still wreck an no-name NPC vessel pretty badly, but more skilled crews would be able to avoid disaster, as it should be.
Yeah for the dramatic option, I’d go with the water steadily drains down rather than a sudden 100 pit instantly appears- which allows the ship/crew a fighting chance to manoeuvre its way out over the edge before the ship falls back and becomes trapped, Id treat it something like reverse Flood (water going to the centre rather than side to side) and also go with 25% capsize chance - thats especially important if the tactic is used against PC controlled ships.