My most stable SE setup, to date.

In my current Survival Endless setup (sorry, no pic), I have (from right to left) three Gloom-shrooms, an Ice-melon and a Cattail. I regularly have to replace the Pumpkin around the Cattail, but I never see anything eating it! Do zombies surface that far back, can Snorkel Zombies swim under the other stuff, or can Dolphin Zombies jump that far?

[Updated 2/1] The configuration shown here is improved, and appears to have solved the problem. I have upgraded the Fume-shroom that was in rows 2 and 5, column 6 to a Gloom-shroom. The extra "backdraft" from that seems to be keeping the Pumpkins around the Cattails healthy.

However, I still don't what was eating them, as I have yet to see any zombies make it that far down the pool.