Lotad turns into a Lombre which is pretty much a water Pokemon and being very good. Another is Marill and Corphish. (There's also of course Kyogre, but he's a bit harder to find and catch)
Kyogre
Pokemon you can get with the good rod are: TENTACOOL, MARILL, MAGIKARP, CHINCHOU ( under-water ) and CORPHISH.
Sapphire is better, but yes, emerald is good.
In Emerald, Pokemon can be found in Grass, Water, and Caves. There are 385 Pokemon in total.
You can.
Snorlax
water, leaf, fighting, psychic, electric, there are a few good normal Pokemon too. if these are not available try and stick with electric and water
The Good Rod in Pokemon Emerald is found on Route 118. If the player uses Surf from Mauville City to cross Route 118, there is a fisherman on the other side of the water will hand over the Good Rod.
Rectha
Linoone (It can learn nearly all of the HM's)
you can use a water stone on lombre
Well, there isn't really any ''good'' water Pokemon. Legendaries are always strong, and a great one is kyrogue, which you can get in Pokemon emerald. It really all depends on which moves the Pokemon knows