you could easily do Tet Paul and then snorkel at Jalousie - the Tet Paul trail is an hour walk/hike tops including time for pictures and just gazing at the amazing views. BigSteve is right in the times and prices.
Personally, if I had only one day, I would go with Jalousie and Tet Paul. The Pitons are a world heritage sight and simply amazing, absolutely iconic sights and what makes the island famous. Definitely a must see on St. Lucia.
Pigeon Island is lovely, but he snorkelin and beach are much better at Jalousie. While it is expensive to get to, the prices are for up to 4 people, so maybe you can share it. Tet Paul is only about 5-10 minutes past the turnoff for Jalousie, so I would go there 1st, then go for snorkeling. You won't be disappointed.
Both places are lovely, you wouldn't go wrong with either one.
Enjoy your visit!