This is a major problem for many casters on land as well. They are
convinced they need to see their back cast in order to be able to
execute it properly. This is not the case! But you will only realise
it if you close your eyes ( on the back cast!). Concentrate entirely
on the feel. Try this at various distances. In a fairly short while,
( although this varies considerably from person to person), you will
know how it must feel for certain line lengths, and only need to
glance over your shoulder to see obstructions etc BEFORE you cast, not
WHILE you are casting. This will improve your casting immediately.
It is also the only way I know to wean people off watching their backs
casts, which causes them to twist their bodies, and ruin their
tracking. You can lean your body backwards, but DONT TWIST IT.

TL
MC