I think the intention is non-UTs only, but the wording is ambiguous. As written you could technically choose a UT, but your options are going to be pretty limited because it has to have some sort of gold value, and has to be less than something (also with a value) that you took from the island. A 0-Gold coin will always be an option because that does have a printed value.
Can great turtles be moved 5S in a turn by a single set of native canoes?
Yes. Each canoe is given its own move action, so you can apply some or all of them to the turtle.