The remark about VFR vs IFR is interesting - an important part of IFR which is often not used....below 3000' clear of cloud you are exempt from 1000' above the highest object - therefore if flying in VMC under IFR or VFR the height you can legally fly at is the same.
(as an aside - it is also often misunderstood that once above 3000' it is very easy for the average pilot to become IMC even though they are actually flying *visually*.)
The only difference therefore is IMC on an instrument approach. Minima are defined by the procedure and personnel licensing (IMC/IR). Additionally you have the approach ban as well.
However, I havent ever really investigated the legality of descending below MSA on an unpublished approach...I personnally would have thought that it wasnt allowable - but I cant remember if there is an exemption for an approach - I will certainly check the wording in UK AIM tonight!!!
Just a thought but as we are talking MSA the fact its over the sea is not entirely relevent - as the landfall would be within the catchment area for MSA calculation.
Additionally NOTHING below 300' makes it onto our charts - that is not just 300' agl but 300' asl!!! Thus making it at least 1300' agl - even over the sea, as you would have to allow for an object up to 299' on the beach, or 1mile out...