Archelon

A friendly archelon is almost always a mount or a guide given by a coastal NPC: a sea-druid's bonded companion, a tritons' ferry beast, a fishing village's revered elder turtle that the priestess says will carry the party to the wreck. The archelon has Int 4, no languages, and nothing on its sheet that suggests intent, so the framing is really about whoever vouches for it.

Use it for one specific water crossing or one specific dive. The swim speed of 80 ft. is the gift. A party riding on the shell can cover three miles of open water in roughly twenty minutes of game time, and the archelon can submerge to take them past surface predators or hostile patrols. The shell is large enough for the whole group to sit on with a Medium-creature crowding rule of your choice, so the scene is the slow approach to wherever the actual adventure starts.

In a fight, the archelon is not a war beast. It will defend itself if attacked in the water, biting whatever gets close, but it does not coordinate, does not wait for orders, and does not understand protect the wizard. Treat any combat round it spends with the party as a beast acting on instinct: closest threat, then disengage and dive. Tell the players up front that the turtle is a ride, not a soldier.

End the relationship at the destination. The archelon drops the party at the agreed shore or wreck, looks at them once, and slides under. If they want it back for the return trip, that is a separate negotiation with whoever lent it to them.

An archelon is an underwater ambush in a calm bay. AC 17, 90 HP, swim 80 ft., and Stealth +5 with Passive Perception 12 make it an apex predator that hides under the surface, lets the boat get close, and decides whether to bite or to drag. It can breathe air and water via its Amphibious trait, so it hunts above and below the tide line.

Open in deep water with no warning. The archelon's swim is 80 ft., the party's is whatever a Medium humanoid can manage in armor, which is almost always nothing. If the players are on a boat, the first sign is the boat lurching sideways as a head the size of a horse comes up over the gunwale. If they are in the water, somebody is already grappled in your head before the first initiative roll. When it engages, it makes two Bite attacks at +6 to hit, dealing 14 damage per bite (3d6+4). Use a generic shove or grapple from the basic rules if you want a tug mechanic, but call it a shell ram or a beak grab so the table can picture it.

Once it has a target underwater, the archelon's job is patience. It dives, it waits, and it surfaces somewhere new. Walk speed 20 ft. means it will not chase onto land except for a few feet at a tide line, so a party that makes the shore is a party that wins. Make them earn the shore. Sahuagin allies, kelp beds, distance to the nearest rock all count. Retreat is easy: the archelon was hungry, the meal got difficult, and the bay is large. Once it is bloodied or the party has visibly stopped panicking, it sinks and is gone.

Have one player roll a Survival check at the start of the encounter. On a success, tell them the water has gone too still in the wrong way. Give them one round to act before the head comes up.