The Lifegaurd CM's at the resorts do wear swimsuits under their shirts and shorts. It's both for sun protection, and so they aren't running around "half naked" to the guests who do not use the swim pools.
Guests at the Water Parks are obviously going to wear swimsuits as it is, and aren't going to offed guests, where at the resorts, not all guests use the pools due to whatever reason. If they are just walking through, they may not want to see all that skin.
Just what i was told.
And like it was mentioned, the CM costumes are very themed for the area, and/or department they are in. In the one "department" I worked in, for the 3 locations I worked, I had 3 different costumes. They are made to stand out and look obvious while also keeping a theme to the area they belong to. Then there are also "global" costumes that stay the same for that role everywhere on property. This includes security, parking, first aid and maintenance. Security, Parking and First Aid keep the same costumes property-wide so that they are the same everywhere. You always look for the "police looking person" - and not the "Jungle Safari guard" in AK, the "Storm Trooper" in Epcot, ect. It stays the same so that you always know what you're looking for.