Swim as intensively as you like, just out eat it. Use a tracker to get a good guess of how many calories you burnt and eat that extra- if you struggle then I'd go for nuts/peanut butter or drinking it.
It's low impact, you're bulking and people are capable of more training than we often assume. It might not be perfectly optimal but the odds of it being a problem are pretty low and if you feel it is then you just reduce the swimming a bit. Also, for your health, cardio is a pretty good idea