the storyline is fairly easy to understand

I watched this the other day and I thought it was really good for a B-grade movie, I thoroughly enjoyed it.
Although alot of people thought the storyline was confusing, but for me personally it was fairly straightforward and easy to understand. So I decided to explain what happened.

So, 6 people go fishing in the Bermuda Triangle, then a serious of mysterious things happen and the ship 'Queen of Scots' which mysteriously disappeared in the 1930's in the triangle suddenly reappears. The group boards the ship in order to find spare parts to fix Morgin's boat 'Bluejay', initially not realizing that the ship is actually haunted by the ghost of Reginald Barrow; an actor from the 30s who was originally a passenger on the ship, he was a psycho and murdered his family on the ship, and perhaps many other passengers too. The ship is also haunted by the ghost of Reginald's son (the same boy who tried to drown Julia while she was swimming & Gus had chased after him on the ship).
The Bluejay mysteriously disappears leaving only the boat's dingy, while the group is on the ship, Stu becomes possessed by the spirit of Reginald and eventually starts going on a murderous rampage. When Tommy & Charlie (Olivia) finally escapes from the ship on Bluejay's dingy and was finally found by patrols, it was later learned that the Bluejay was missing for 4 years, even though it felt like the events happened in only a day or so...implying that the Triangle is actually a wormhole, and that time travels much slower inside the Triangle compared to the real world, it's part of the whole mystery of the Triangle.

Cabin 116 on the ship is featured significantly in the movie because that was the cabin that Reginald was staying in, and he supposedly is the antagonist of the story. And why did Reginald possess Stu? Probably because Stu is so money-hungry & he did find the key to cabin 116 in the sea while he was snorkeling.
I think the voodoo ritual that Tommy & Gus stumbled upon is tied to the whole misfortunes that happened to the group, the voodooist who performed the ritual was mad at Tommy & Gus for disturbing them and put a curse on them, which probably caused all the bad things to happen to them.

As for why both Tommy & Charlie had the same bad dreams about cabin 116 even before Tommy & his friends went on the holiday, Charlie obviously had a 'gift' of dreaming about the future, and Tommy probably does too. Which is why they both foresaw in their dreams that something bad was going to happen.