Probably a silly idea, and only for if no good lock ideas pop up (as I don’t have a good idea short of drilling a hole in it and installing some kind of lock).
However, if nothing else comes up, and banking on the possiblity of any thief being (1) stupid, and (2) just a potential theft of opportunity, put down an outdoor throw rug runner over the top of that spot on the floor running most of the length of the boat.
I am envisioning an outdoor runner that is long but narrow. My guess is someone would be just peaking, and if they don’t notice an easy opportunity, probably would not know or think to check if there is in floor storage under the through rug. Also, if a longer runner, they would be most app to peak under it at either of the two ends, and your in floor storage would be more in the center. Thus, even if they peak, they would most likely just see your normal floor vs. the in floor storage door.
Again, just a diversionary idea if no good lock ideas come up.