Tide charts for December in Kennebunkport
It seems that low tide was at 10:12 PM on wed night and high tide was the following morning at 4:36. Around there, sandbars often appear at low tide and connect various islands. They rapidly disappear as the tide comes in.
When you grow up on the water and around boats, especially on the NE coast, it's pretty normal to just "take a kayak and toot out to that island over there". When you're young, you'd think doing it at night in the winter is a crazy and fun thing to do.
So if they left the house after midnight and went out via a kayak or something and headed out to the islands, they could very well have parked on a sandbar. It was a clear night, little to no wind and half a moon. It was probably deceptively quite out there. They were going against the tide at the time (which was already starting to come back in) and perhaps when they landed on sandbar, they miscalculated time and as the water overtook the sandbar they either lost their kayak and figured they'd wade/eventually swim to the nearest island.
Once on the island, wet freezing clothes would come off, on the rocks. It seems they both made it to land.
Perhaps they thought, after waiting for first light, that they would try to swim back - but at that time, the tide was already on it's way back out. The cold water, the distance, the tide fighting them would have been no match.
I think this scenario is more likely than someone planting clothing there.
This doesn't, however, explain why, if they tried to swim back, they would leave the life jackets on the shore. Perhaps they thought they were faster swimmers without them?