Pokemon Heartgold/Soulsilver:
Surf along certain routes and you will find nothing but Tentacool and Tentacruel in the water:
Route 26,
Route 27,
Route 34,
Route 40,
Route 41(plus Mantine in Heartgold only- 2 SpDef EV's)
Also, you can find them in cities or other significant locations by surfing, too:
New Bark Town (off the shore on the right);
Cherrygrove City (off the shore on the left);
Olivine City (off the harbor to the south);
Cianwood City (off the shore farthest north);
Cinnabar Island (around the island);
Pallet Town (off the start of the inlet to the fence)