What, you mean like a trampoline?

(sorry!)

I've found that the best thing to use is a small amount of silicone grease. Quite a few people use bath sealant or silicone mastic but I don't like it because it's sticky, doesn't compress as well as grease and seems to get everywhere and jams things up when it solidifies. The grease just forms a nice watertight(ish) seal and is easily removed and cleaned off.