Lapras?
Correct!

-This Pokemon has two types - Water and Ice.
-It does not evolve - According to Bulbapedia, Lapras does not evolve.
-Despite its size, don't expect it to eat you - It's huge, but due to its kind nature, it is very unlikely that it will eat you.
-I said "don't expect it to eat you" in the third clue because this Pokemon is nice - See third clue.
-This Pokemon can swim very well - Lapras is a super strong swimmer.
-It is a Gen. 1 Pokemon - From Kanto.
I'm honestly surprised that took so long, but hey, pretty much everything I did before Lapras was pretty easy. I guess this is a nice change for once.