Dorsim

Dorsim is a mod that allows players to ride Dolphins. Though it is unfinished and the player cannot get off the Dolphin whether attacking or using the dismount key.

The ability to dismount Dolphins was planned but never implemented.

Speculation/forget if it's possible to use TNT or have mobs attack the player to be able to get off the Dolphin.

This is a Rift mod.

This is a minimal mod with the simple goal of giving you, the player, the ability to ride the dolphins.

Apparently Mojang did not include this in vanilla because it would constitute or promote animal cruelty. But what they missed was that these dolphins are minecraft dolphins and as creatures of this flat blocky land, they aim to be the new rulers of the dominion of this sea. Unfortunately for them, you were there first. You were there when then were no seas to rule, no clouds to soar among, no trees to fell, no crops to cultivate, no undead to slay, no red to stone, no axe to pick, no lapis to lazuli, and certainly no water mobs that ee-e-e-e-e-e-e-e-e-eeed at you while you charted the unknown oceans in your wooden shoebox. You were even there when there was no where to there. This is your world. It bends to your will. So you can ride those dolphins if you want to, but only if you ask nicely.

Minimal effort was put into this mod so don't expect much. You don't need a saddle and it's basically like riding a pig. You can't control where it takes you so you might drown eventually. Also you can't dismount. I'll implement that some day lol.