Easiest item/spell/other for making a Tendriculos aquatic?

Question

Building an a character that uses the Arms & Equipment Guide rules for having a Tendriculos as a mount (basically, raise it from a seed and use alchemy to turn off its digestive acid, then ride in its mouth). Looking for the best way to make it suitable for aquatic adventures, be that item, spell, or something else. The character's tentative class is Artificer for the flavor of 'mad scientist experimenting on self and pets' (with Able Learner so I don't have to spend so much on Ride and Handle Animal), but I'd be willing to switch that.

By "easiest", I mean that I'm looking for something that can be gotten/created early, but will still do the job of letting the Tendriculos breathe underwater (in a perfect world, it would also grant a swim speed).

Answer 1

Pearl of the Sirens

A pearl of the sirens might do what you need:

The pearl enables its possessor to breathe in water as if she were in clean, fresh air. Her swim speed is 60 feet, and she can cast spells and act underwater without hindrance.

It needs to be held in hand or close to the chest to work, and looking at the tendriculous, these anatomical terms may be a bit of a stretch, so check with your DM if they are fine with it. The thing has at least 2 tendrils with which it can grab, so it should be able to use one to hold the pearl.

Alternatively, you could try things like a ring of swimming, or more costly but better, a ring of freedom of movement combined with a ring of water breathing (not from the DMG, Magic Item Compendium, p. 128), but it will suffer from similar anatomical reqirements.

Body slot rules

The Rules Compendium says for Nonhumanoid Creatures (p. 84):

Armless Creatures. Creatures that have no forelimbs don't have the arms, hands or rings body slots.

The tendriculous has forelimbs in form of its tendrils, so it should probably be good on the hand body slot.

Fingerless Creatures: Creatures that lack flexible digits or extremities lack the rings body slot.

For the rings, it will come down to if the DM considers the thin end of the tendril a flexible digit. They look pretty flexible to me.

Answer 2

Magic armor and a crystal of aquatic action

You can get whatever kind of magical +3 armor you want, but if Funguy the tendriculos is an otherwise normal tendriculos (and not trained for war—see here) then +3 keel (Stormwrack 160) nonhumanoid Large studded leather (Player's Handbook 123, 126) (9,300 gp, 42 lbs.) will have no armor check penalty. Then attach to the armor the augment crystal greater crystal of aquatic action (Magic Item Compendium 25) (3,000 gp; 0 lbs.).

The crystal negates armor check penalties on Swim checks and grants a swim speed. Also, the crystal causes the creature to suffer no movement or combat penalties underwater in a way that's explicitly like a freedom of movement effect. And, almost as an afterthought, the crystal grants the creature the ability to breathe underwater.

At a combined total 12,300 gp, the pearl of the sirines (Dungeon Master's Guide 263) (15,300 gp; 0 lbs.) is comparably priced, and there are certainly advantages to the pearl (e.g. cast spells, flat swim speed), but as an artificer, you can easily work your way up to this combination, likely being able to improve both the magic armor and the augment crystal as you advance in levels. Funguy'll need to armor up anyway, but you may want the pearl.

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Note: This answer assumes that the tendriculos must possess a swim speed and the ability to breathe underwater constantly. Making the tendriculos aquatic by employing potions, scrolls, or wands is relatively inexpensive for an artificer although, obviously, only temporary.

Answer 3

Cheap: Beruto of the Carp Dragon

The cheapest item viable for your purposes is the beruto of the carp dragon, a waist-slot item found on page 99 of Dragon Magic. A tendriculos has a waist slot, so it can wear this one. The beruto costs 2,200gp and grants a swim speed equal to land speed and the ability to breathe water, but takes away the ability to breathe air. So long as you take the belt off your plant friend after leaving the water you'll be fine, but it may cause issues in edge cases where you're leaving and entering water during a combat.

Luxury: Hathran Mask of Water Breathing

The Hathran mask of water breathing (Unapproachable East p. 57) costs 3,000gp and grants a constant water breathing effect. A lesser armor crystal of aquatic action (MIC p. 25) costs 1,000gp and can be attached to a suit of +1 armor (another 1,000gp) to grant a swim speed, for a total of 5,000gp spent (plus whatever the mundane armor costs); still quite cheap, but not as cheap as the beruto.

On Handle Animal

Note that, as a tendriculos is sapient (Int 3, the lower bound of sapience), you do not have to worry about teaching it to use the item or using the Handle Animal skill's tricks—and in fact cannot use those rules for directing it (not without a houserule, anyway).

What you need instead is to teach the tendriculos common (unclear if possible; it's not listed as knowing languages normally) or to give it a pearl of speech (600gp, MIC p. 118), and then you can communicate with it normally to direct it. Page 23 of the Dungeon Master's Guide directs your DM to decide if they want to have a sapient mount act on your initiative or your own initiative, but it will likely be simpler than using the Handle Animal Rules regardless.