I didn't know 3.5 had seaponies I'm a bit scared to, but I will have to look them up.
It probably depends whether you are calling the ponyville mer ponies G3 or 3.5 but I always class them as 3.5 to be honest as thats when they kind of synced up for release plus they are all Core 7.
I think other people either dont class them in either gen or consider them more G3.