From a purely songwriting standpoint (morality aside) the word "ass" fits the lyric perfectly ("...my ass in the sand") good alliteration and it sings wonderfully. In the context of the song it completes the thought of having absolutely nothing else that the singer has to do -"Life is good today..." Its irreverence points up the "bubba" mentality of the singer, hence the popularity of the song.
You know, in the grand scheme this is pretty innocuous especially up against something like Pink's "It's You and Your Hand Tonight".
I concur with Sheila Davis (The Craft of Lyric Writing) that there is a need to be mindful of what your song/lyrics will convey when they are targeted at certain vulnerable groups within a society such as impressionable youth, etc.
In this regard, it's just Jimmy Buffett - a tad over the top. BTW, didn't he write "Why Don't We Get Drunk and Screw"?

Marty -
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