I do think that if you're going to do this some experience
with instrument flying is essential as you frequently will
lose your visual horizon completely as the sky and the
sea merge in a dull-green haze.
I agree completely. If you can't see land AND you don't have some sort of horizon, even though you can see the sea and you're legally VFR, it's quite disorientating and you sometimes find you need to fly on instruments.