This is how we've been playing the game to make dolphins a little more useful and also make the monsters more predictable (we feel the game was a little too chaotic before).
Single Blue Dice Variant:
1. Only roll the single blue dice with the creature icons 2. If a monster is rolled (Shark, Whale, or Serpent), they move as far as they can normally move under the base rules. 3. If a boat is rolled, an unoccupied boat moves up to 3 spaces 4. If a dolphin is rolled, it can move up to 3 spaces. If it started it's move paired up with a swimmer, it can bring the swimmer with it up to 3 spaces. You can make a dolphin abandon another player's swimmers.
When the dolphin tile is drawn, you can keep it in your hand as per the base rules. When you choose to use it, place an unused dolphin meeple next to the swimmer it will be carrying, then move them both up to 3 spaces. If there are no unused dolphin meeples, then you may choose ANY dolphin meeple on the board, even it is means it will abandon its swimmer.
Like your suggestion but after playing a game with these rules the dolphins seem over powered. I suggest making the dolphins susceptible to sea monsters (makes sense if you think about it). So dolphin tiles allow you to place a dolphin anywhere and move one person with it up to three spaces. Rolling the dolphin allows you to move a dolphin already on the board three spaces (with a person attached if they were already on the dolphins back). And the dolphin protects one person from shark attacks. What do yall think?