Guys, help with a weird fishbox on my new (to me) Robalo

[MakoMac]

Guys, I have a Robalo 1920 cc that has a front floor locker. Some call it a fishbox, but as it has no water inlet valve and only a drain, I just call it a fishbox. Now the problem... I am pretty sure the drain comes in from the water line port on the port side of the boat. The boat is new to me, but the previous owner says it held water in it and he let it stay in there so it would help with planning. Well, I don't want a box half full of water on my boat so, I used a shop vac to blow, and suck for 15 minutes and got just a trickle to come out. When the shop vac was on blow, it would blow good into the box, but when I would switch it to suck, it would only trickle. Its like there is a valve somewhere, or this is a one way water inlet. but why the heck would it be like that?
Tomorrow i'm installing a new Humminbird helix 7 so I will have the console apart and the floor panels up and actually lay eyes on the drain.
But, anyone know what the issue could be here?

 

[tpenfield]

Is there a screen on the drain fitting in the 'fish box'? If not, then there may be a clog that cannot go out past the thru-hull fitting.

You could try to force water (garden hose nozzle) in at the thru-hull fitting, or apply suction at the drain in the fish box.

 

[Scott Danforth]

My guess there could be a check-valve installed backwards

Coffin boxes used as an ice chest have a drain line that allows waste water out

Floor boxes are generally storage with the same type of in-line scupper check

 

[MakoMac]

so I dug deeper today and removed the batteries, removed the deck cover and low and behold, I find a dead grinder pump that the old owner swore it didn't have. Looked it up.... $230.00.
oh what a wicked web.... lol