The top of the pylons are sealed with plugs of closed-cell foam, with something like silicon sealant around the edges. The foam plugs deteriorate in time and begin to leak badly. Sections of pool noodles can be cut off and fitted into the pylons quite nicely. A little silicon around the edges makes them completely waterproof. The front pylons have a small vent tube to relieve pressure on the hull. Work around them and don't plug them accidentally.