I had the same problem on Magdalena about 5 years ago. Our ice box had an S trap below the drain. Water would not drain from the box because somewhere below the drain was a clog.
Plan A: At first I tried to access the space under the box (port side, next to the engine compartment). I tried all manner of attempts to find access without cutting wood, and all failed.
Plan B: So I thought I'd pull the drain up and perhaps get the trap and hose up into the box. Well, as the drain came up, the trap fell down onto the hull, about 10" below the bottom of the icebox.
Plan C - part 1: Ok, the only thing I could think of was a surgical operation. I had a 2' flexible grabber and managed to grab the trap and bring the hose up, but the trap was bigger than the drain hole. I managed to tie some string to the trap so I no longer had to hold it with my fingers. I then was able to remove the hose from the trap (with fingers and screwdriver). I then pulled the hose up and found the drain problem to be a kink in the hose instead of an actual clog. Luckily, the kink was close to the end of the hose, so I cut about 4" of hose where the kink was and brought it up through the drain hole.
I then figured I'd reconnect the trap, but that was just not going to happen. So...
Plan C-part 2: I decided to get another drain from the hardware store that the hose would fit onto. But before I did anything else, I had to immobilize the trap so that it wouldn't roll around while sailing. I got a rectangular piece of wood that could fit through the drain hole and slopped some 4200 sealant on one side of the wood. The string was still attached to the trap which enabled me to bring it up to working distance and smacked it on to the sealant on the wood. I let it sit overnight, then pulled the trap up again and then I spread sealant on the other side of the wood so that I could attach that side to the hull. Then I lowered the wood (and trap) down to the hull, got a stick that allowed me to press the wood down onto the hull. Again I let it sit over night. The next day I used the stick to try to move it. It didn't move so I then, with some more surgical moves with fingers, I got the hose up through the drain hole, attached it to the new drain, then lowered it back in and sealed the drain to the box.
Water flowed like a gusher again, and I have never heard any noise of the old trap and wood bouncing around the hull.
If anyone chooses to do something like what I just described, I should mention that I spend so much time bending over the icebox for two days that I was not able to walk upright, but went around like I was looking for spare change on the ground...
