MSA over the sea
Simple, really... what's the MSA over the sea?
If you wanted to let down to an airfield on the coast without an IAP, and could let down over the sea, how low could you legally go? (Ignore for the moment the question of the legality of home-made IAPs, which might or might not be safer but where descent below MSA is involved - that's a separate argument.)
Do the charts as usual show all obstacles above 300ft?
Tim