Phil: If it is the old-style cast hatch – which are very good – it could be the seal between the hatch and the deck, or the seal on the hatch itself. For bedding the hatch, your idea of wetsuit material is excellent. This is what we’ve been doing for years on all deck hardware. Otherwise, you need to replace the hatch’s gasket between the opening faces.

One thing which helps is to have breakwaters around hatches, in your case from teak, so they don’t get hit by wave impact. This is standard practice for us also.

If you have a copy of Offshore Cruising Encyc. aboard there are some photos of breakwaters. – Steve