[robin_reala]

people living on islands should be sufficient evidence of boats

Historical sea levels were wildly different at different times, so not necessarily. For instance, the British isles were settled at a point when it was a part of the mainland: https://commons.wikimedia.org/wiki/File:Doggerland.png

[MarkusQ]

Certainly. Land bridges are also a thing. As is swimming, in some cases.