You can’t think of a better option than “riding” the seabeast, though. You briefly consider guiding the tunneler in that direction, but it would be harder to guide it precisely enough. It would need a lot of distance to make a big turn, and you might make a mistake and wind up in a worse position. At least if you’re sending it towards the desert, that’s a wide target and you don’t need to be very precise.
By comparison, the seabeast would be fast and relatively easy to guide. So that would speed up the meeting, even accounting for the fact that you’d have to make two trips. Which means options you’d dismissed because they wouldn’t work for very long might be more viable.
First and foremost is calming down the tunneler. You can’t really do that directly – the easiest way would be to mentally link with it and broadcast calm thoughts, but you already tried linking. The only thought you have is a calming sound.
Such as the ocean waves. You don’t even need to craft the sound, just transmit the real thing to the tunneler. That’s not very hard, you can just create two sound relays. You make one near the surface of the water, and then head down to make the receiving relay in the tunneler’s ear.
Then you observe its reaction. If it calms down at all, you can probably make the trip safely.
It does seem to be calmer, but something about its reaction strikes you as odd.
It mimics the rhythm—by sending out pulses of its own, the pulses disturb the marine life and complicates things
It becomes agitated, misreading the signal as a threat, the positive thing is, you have its full attention
It calms, but the calming worked too well. Now it won’t move at all, and its massive body is starting to sink in deep