Riggs looks like he's showing off his fashion sense to Murtaugh. Is he telling his buddy that there's no need to change clothes every day? Especially the shirt? Maybe he just has a closet full of the same shirts because he likes them?!
We can't even imagine how they ended up in a swimming pool. Why does Murtaugh have his gun drawn? The people in the background don't look too concerned about whatever is going on. Maybe they're at another pot party.