If I become an LSA instructor in airplane single engine sea, can I
give dual training and log book endorsements to a private pilot that
is trying to add the class of single engine sea to his certificate? I
looked at the regs and it looks like I would be technically be giving
instruction for someone going after an LSA single engine sea class,
but I think there is an provision that training towards LSA pilot
level, can be use later to go for a private level certificate.

I personally don't see why sea plane training at the LSA level can be
used to get a single engine sea class added to a private. If I held a
regular CFI, I am very sure I could provide private level training in
an LSA aircraft.

Danny Deger