they find human-made tools (which they can assign dates to) on islands distant from shore.
Their only explanation that they are hypothesizing is that humans traveled there across water. Do you have a better idea? They're not saying they found boats, they're saying there is clear evidence of human travel.
Focus on the evidence there is, not evidence you would like to see. What do you hypothesize that fits the evidence?
That the tools were moved to Crete way later than when they were made. Obviously, there are no proof of that either. I am not saying it is impossible Neaderthals got there by boat-ish means. But recycling seems like a possibility not moving back seatravel too far into the past.
"Sail" can mean any travel over water by ship. It's one of those weird words that can be really confusing even when used correctly but out of the context where that portion of its meaning is customarily used.