Well, the lake would be harder to swim as far as in the ocean simply because of the bouyancy fact. dangerous animals fair very little in ocean simply becuase of the animals that are the most dangerous in the ocean (jellyfish, sharks ect.) most of them aren't all that dangerous, and the ones that are aren't very aboundant. But, depending on the ocean, there are many islands, and chances are if u are going to get stranded in one of the two , u are either in a boat, or on a beach which the beach wouldn't really be dangerous lol. The boat on the otherhand, is what imo makes one more dangerous then the other. The majority of boats taken out into the ocean a ways are rather large and therefore do hold emergancy things such as water rafts and the sort, while boats ment for lakes are usually smaller, therefore there is less of a chance of having something to get to shore with, besides a life jacket.
so really i'd go with lake overall, but they really aren't very dangerous at all when you think about it.