Fully evolved Water types only, in order of obtainability, from earliest to latest (not taking Hidden Grottoes into account). If you wanted "in order" to mean something else, you probably should have made that clearer.
Samurott
Golduck
Azumarill
Simipour
Vaporeon
Swanna
Basculin
Floatzel
Jellicent
Alomomola
Pelipper
Mantine
Octillery
Starmie
Wailord
Walrein
Lapras
Corsola
Dewgong
Crawdaunt
Seismitoad
Carracosta
A few notes:
- The Water types that don't require Surf or the Super Rod are Samurott, Golduck, Azumarill, Simipour, Vaporeon, and Swanna.
- Crawdaunt, Seismitoad, and Carracosta are locked behind the postgame.
- There are some other Water types in BW2 that aren't in the Pokedex, such as Politoed, Gorebyss, and Relicanth, but all are locked behind the postgame and as such aren't going to be very helpful.
- Keldeo exists, but is unobtainable through regular gameplay. You'll need to use the DNS Exploit if you want it.