It's possible you're thinking of the "Underwater" feature in Pokemon Crystal which allows you to explore the ocean depths. Here's how it works:
1. Obtain the Dive Ball: You get this item from the "Dive Ball Man" in Olivine City after you've finished the lighthouse puzzle.
2. Learn Dive: Teach a Pokemon the "Dive" TM which you can find in the Ilex Forest.
3. Use Dive: You can use the "Dive" move while standing on the ocean surface. This will take you underwater where you can explore and battle new Pokemon.
It's also possible you're thinking of "Sea Skim", another move that lets your Pokemon travel along the water surface. However, this move isn't in Pokemon Crystal.
If you have more questions about specific features in Pokemon Crystal, please let me know!