just use surf on almost any part of ocean
tentacool
Like all games Tentacool evolves into tentacruel at LV30
Use Surf on Route 205 and you'll easily find a Tentacool.
it is Tentacruel
In Pokemon Diamond, Pearl, and Platinum you can find Tentacool in almost any large body of water. Once you get the move Surf from the Pastoria Gym go back to Sandgem town. You have to teach a Pokemon surf first, pretty much all Water type Pokemon can learn it. Then go down to the beach and use surf in the water. Swim around a bit and you're bound to run into a Tentacool.
in the national pokedex it is flareon which i have finished the national pokedex in the first pokedex it is tentacool in platnium
The Pokemon are Chatot,Tentacool,Tentacruel,Floatzel,Gastrodon west sea,Shellos west sea, Finneon,Gayarados, Lumineon,and mr.mime
Tentacool is 072 i guess. im not so sure though cause ive seen it but everyone says its him sooo.
In almost all bodies of water like for example the route between canaclave city and jubilife.
if you have surf then anywhere with water
In the National Pokedex, it is Tentacool. Should be the same for Platinum.
In RSE you can find Tentacool on Route 103.