Sirius was on the job. Which was to say, he was sprawled back on the sand, ready to jolt into action if someone should start yelling and flailing their arms for help. It didn't happen often – or at all really – but someone had to pretend to be there for the safety of the swimmers. So long as it kept filling his bank account with money, Sirius didn't much care about the lack of damsels and gents in distress. When he wasn't saving lives, he was exploring the beach and working on a fantastic tan.
He might not have even noticed Bellatrix walking along if she hadn't announced her presence with the question.
"This is my job, ta very much," Sirius informed her, lowering his sunglasses as he threw back, "Shouldn't you be hiding from the unhappy mobs?"