Unidentified White hulled vessel, possibly in the Boston area in the 1940s. The harbor tug is named IWANA and it was in service in Boston up until 1942. What is the name of the mystery white vessel? I’m curious if it possible it was a naval training vessel or a USCG revenue cutter. There are sailors on board with white uniforms and it is an American flagged vessel.
On reading the question I had the hunch, "looks like a US Presidential yacht", and googled "presidental yachts of the united states" and found a page with a photo that looked very much like the OP's picture, naming it as the USS Mayflower. The overall appearance of the ship seemed to match, especially the placement of portholes.
Following R. Leonard's suggestion to look at blow-ups of the photo, I can almost read the name on the stern as "Mayflower", and see overall match with trustable pictures of the Mayflower, such as this and that.