Marina tossed the wetsuit back onto her boat and dove into the water. Having gotten used to swimming with an artificial monofin it wasn't much of an adjustment with a real one. If anything the flexibility of her tail instead of legs gave her extra maneuverability. The tail was powerful enough that her webbed hands weren't needed for more than a little steering.
So focused was she on swimming it took awhile before Marina realized she hadn't come up for air since she dove in. She must be breathing water, but that seemed relatively unsurprising after having been turned into a mermaid. Marina reveled in the freedom of diving without having to worry about her air supply. She lost track of time.
Marina surfaced to get her bearings. She hadn't gone too far from shore or the dock with her boat, but the day was getting late. She was hungry and, as much as she enjoyed swimming, she was worn out from the day's adventure - first her treasure diving, then her transformation and subsequent swim.
Not wanting to spend the night in the ocean, she considered her options: return to her boat and try to get help; go ashore at the nearby beach and try to get help; find a secluded location to spend the night ashore and find some food along the way.