I have not been to this island but have spent a fair amount of time on the island the tour leaves from, Caye Caulker, and have done snorkeling trips in the area, so I've been following this case.
Shark/gator attack or rogue wave - highly unlikely. The sharks in the area are non aggressive and attacks are incredibly rare. The island is 20 miles offshore and doesn't appear to have much vegetation or habitat for gators. Belize beaches do not really have waves as they break at the barrier reef.
However, there are plenty of other dangers in the water. For one, the Belize cayes are full of coral which is incredibly sharp - it's easy to cut yourself and lose enough blood to get weak and disoriented, which could lead to a bad outcome even in shallow water. And this island is very close to the barrier reef which is surrounded by very strong currents. I'm a strong swimmer and thought it was going to slam me into the reef a few times while snorkeling. It's very common for ocean-inexperienced people to not realize how close to shore the dangerous currents start and assume they are safe because the water was so calm a few steps back. Once caught in a current, you can get slammed against a reef, disoriented in the dark, exhaust yourself trying to fight it so you can't tread water to wait for help. Honestly, it would be very easy to die in the water even if she wasn't intoxicated.
I know the family said she doesn't swim, but she was going through a lot and maybe she decided to face her fears - especially if she was under the influence of alcohol, a manic episode, prescription drugs, or whatever else caused the erratic behavior. (Bipolar disorder wouldn't onset at her age, but it's not unheard of for people with Bipolar II to go undiagnosed, and a manic or hypomanic episode would make her far more likely to take all kinds of risks like swimming and drinking.) Or maybe she had been getting over her water fears already, since she had been in Belize for a month? Her instagram had a picture of a waterslide into the ocean.
I don't think it's suspicious that a guard would be telling people not to walk around at night. There are many ways that drunk people walking around in the dark surrounded by water could hurt themselves and they don't want to get sued or have to interrupt the trip to take someone to the hospital. I don't see this as evidence of anything more nefarious. I also just don't think that randomly witnessing a drug exchange makes sense - there are plenty of uninhabited islands with more vegetation cover for them to meet on, and it's unlikely that she would be able to accurately describe anyone she saw in the dark, so killing her and drawing American attention would be riskier than letting her go.
I feel like a water accident is the Occam's Razor explanation. Even with the fear of water, an intoxicated woman going for a swim at night is far less far-fetched than what a lot of the other theories require. But if foul play was involved, the two most plausible scenarios I can think of are targeted killing (related to her boyfriend's business dealings in the area, or any other things they had gotten involved in), or she got into a physically vulnerable position with another person where they were able to kill her without her having a chance to make much noise (consensual sexual encounter, going for a swim together). Maybe an affair with another passenger who feared she would tell his partner? (Again, it's pretty far fetched compared to accidental drowning imo, and I'm not aware of any evidence they were involved in sketchy things or she was cheating.)