Long ago I tried Trollbabe with some friends. One of them playing to role-games for a long-time with me.

I show them my map, and told them to choose any place. I added thay could choose any place to start, not only named ones, but any small drawing or detail in the map was OK, because I had added some symbols or details here and there with no clear purpose yet.
One of the players chose a big sea-serpent in the middle of the sea that I added just as decoration. Indeed he was really excited about the idea to start his first scene riding the serpent! (Well, Trollbabes cannot start riding anything, that was clear in the rules).
But I never thought about the serpent as a possible place. I was stalled. I just recognized to him that I was not thinking on it previously and we started in another place. However we agreed that riding fantastic beasts could be possible and the sea-serpent was not discarded for the future. He as an easy man, he understood my problem and we continue happily with the new place.

However, I regret so much not to have an idea to exploit that nice player excitement.

I have been thinking again on it. Trollbabe's stakes are easy to create revolving about social conflicts. Between humans and/or trolls in any direction. But I had no idea of what to do with a sea-serpent. Now I see that the serpent is as any other drawing or place in the map. I could have devised something like "Moby Dick", a captain trying to capture the serpent. Or the serpent as magically controlled by someone causing problems to others. Or the serpent only as a faint terrific presence in a social conflict somewhere. Or whatever.
However, I still have troubles to create new stakes-ideas in which the sea-serpent is relevant, specially for a starting Trollbabe who is going to affect only "Personal level" stakes. At least more troubles than in more normal places.
I think that I'm also pushed-back but a preconcived idea: The sea-serpent drawing is in the middle on nowhere, in the far-sea. I think I'm trying to create situations in the middle of the sea, far from any coast or island which is more complicate for me.

Am I just looking at it from the wrong point of view? Any ideas?

That doesn't seem particularly difficult to me, really. Stuff might happen on ships, as you said. And sea-trolls would be just awesome - human whale-hunters on ships and trolls who actually live in the sea. Sounds dandy to me, not much need for land there. Almost anything you could do on land, you could do on sea as well. The sea serpent can then be controlled by trolls, be a target of veneration or fear or whatever, just like a land-based monster.

Going under the waves is a staple of fantasy as well - visiting some sea-troll village on a coral reef sounds fun.

Hi, Eero. Thanks for the reply.

This is exactly where my "wrong point of view" is. You are talking about things I could classify or name as "high" pulp-fantasy. This player was also expecting it (riding fantastic beasts sounds like that). But I don't know why, I'm thinking on ideas restricted by a kind of commitment to a gritty "low-fantasy" setting that lives in the back of my mind. Perhaps it is inherited from other games, from older times.

I really need to work on this. I will try to do the exercise to have a look at specific details on maps, drawings, comics or whatever and generate simple "high-fantasy" stakes or situations. Perhaps it helps me.

Interestingly enough, I don't see sea-trolls as particularly high fantasy. Rather, they are a staple of low-fantasy gaming for me. Might have something to do with water-trolls being pretty common in Finnish folklore.

In the region of Spain where I live, Nordic tradition, and most Celtic, was seen as exotic, romantic or at least different. Indeed, elves, trolls, etc. are more or less considered high-fantasy. Most of us were introduced to them by Tolkien's writings. Faeries and witches by Grimm's tales. About the sea, I don't know if it is due to living on an inland plane, surrounded by mountains, but all sea and specifically under-sea settings are not present in our verbally transmitted stories. However, our fascination for all those traditions is intense.