The other day I mentioned in passing to a friend that I'd been bodyboarding last week in Cornwall. They did a double take, then roared with laughter at what they thought was my joke. I was slightly miffed - why was the idea of me bodyboarding so unlikely? I mean, apart from my often announced detestation of cold water, dislike of active sports, hatred of getting my hair and face wet, my advanced age and general unfitness... It has to be said I've only started recently (a good wetsuit has helped), but to many of my friends it is completely implausible and out of character.
In fiction, you could make this work but there are two areas to watch out for. The first is plausibility and the second is motivation.
The plausibility comes in part from context - the character moves to a new area, gets a good wetsuit, sees everyone else having a good time and decides to try it herself (pretty much my reasons for trying it).
Then there's motivation. Why would a character change their habits? As a writer, this is where it gets interesting. You could use the bodyboarding as a sign that they were capable of changing their attitudes, or perhaps conquering a life long fear of water. It could be a sign of refusal to accept the ageing process. Or an acceptance of ageing in the sense of not being bothered if one's body doesn't look as great as the teenagers on the beach. Perhaps it could symbolise a greater boldness towards life.
Whatever - it doesn't matter what the motivation is for the change, so long as it's there.
In real life we don't usually spend hours examining why we do things, we just go ahead and do them. Characters similarly shouldn't spend hours in contemplating their motives, but the writer needs to know them because if they don't then nothing will appear plausible to the reader.