It isn't a death setence for fighting ability, but it mimics fighting conditions extremelly poorly, and that's the least of your worries when it comes to swimming. It takes up a lot of calories, especially if the water is cold(it burns fat really well bc of this). The worst thing is, it trains your neuromuscular system to move slowly and with little resistance; basically slow twitch muscles all the way, which isn't something you're looking for if you're a fighter. Also, a really famous Judo and TKD trainer in my country told me the slow, low resistance movements while swimming dull your reflexes and reaction time, and this guy knows his shit(he's also a professor at 2 colleges, one here, one in Russia). If you are really keen on swimming for SnC, there's is a way-how I would go about it, but you kinda have to be an advanced swimmer, at least technique wise. Basically, you swim one length of the pool(or a distance you determine) in butterfly in explosive passion, then you swim back to your starting position in relaxed backstroke. Do it in backstroke to relax between butterfly sets; since it's basically the most passive stroke and when you swim on your back you take the water pressure off your chest/lungs. Structure it like a HIIT workout(eg. 5-10 sets of explosive butterfly, 5-10 sets of active rest in the form of relaxed backstroke). Also, make sure to hydrate properly; a lot of first time swimmers forget to drink water and then become surprised when a dull headache appears.
PS Nothing can replace roadwork, as we clearly saw with conor