May in Santa Monica can have a weather pattern called "May Gray" - lots of grey, low clouds that gather by the coast early in the morning. They may be gone by mid morning, but they can linger into early afternoon or later some times. Sometimes this doesn't happen and its sunny all day. Water temperatures in the ocean will probably still be too cold for much swimming, regardless. August for sure is a better time for Santa Monica, but for sure its a worse time for Vegas.
JW Merriott / Le Merigot is the best hotel of those you listed, plus its right on the beach. After that, I'd probably not do Delfina vs. the others - its probably a bit nicer than the others but a bit further inland and in a less happening area.