EDon't mind me. Just hopping into the pool!
My favorite Water-type Pokemon is also my favorite Pokemon, Blastoise. Has been ever since Pokemon Blue. Many adventures have been had with the big turtle, and I will always love him (Mine are usually always male, naturally). Shoutouts to Crusher, my first Blastoise who lives in a copy of Pokemon Stadium to this day, and Ark Royal, who resides in Pokemon Home.
My other favorites are Tentacruel, Starmie, Kingler, Milotic, Gyarados, and Primarina! Honestly would love to use those six on like a Gym Leader or Elite 4 team.
Favorite Water-type trainers are easy for me. Misty and Nessa, naturally the first and latest Water-type Gym Leaders. Didn't mean for it to fall like that, but it did. I love how determined both of them are. Pretty sure if they met they would be rivals, with Misty being the tomboy she is and exploring everywhere while Nessa gives her fashion tips.