Resistance training and cardio

What I'd say is to keep your cardio quite low intensity otherwise it's going to significantly impact your recovery from lifting and impair your strength gains. This is especially true since you're lifting 6 times per week. You might say that going low intensity with your cardio will mean that area won't improve quite as fast, and that's correct, but you can't make maximum gains in all areas of training at the same time. You have to decide what your priority is. If it's muscle building then you want to train that area the hardest and take it easier with the cardio. On the other hand a professional sports person is more likely to hit the cardio hard but not train weights so hard that it compromises performance in their sport.

So you've got the right idea by doing your cardio after your lifting if you want to prioritise lifting. And a calorie surplus will optimise the strength and size gains, but try and not go overboard with the surplus, or excessive fat gains will start affecting your swimming performance.

What I'd say is to keep your cardio quite low intensity otherwise it's going to significantly impact your recovery from lifting and impair your strength gains. This is especially true since you're lifting 6 times per week. You might say that going low intensity with your cardio will mean that area won't improve quite as fast, and that's correct, but you can't make maximum gains in all areas of training at the same time. You have to decide what your priority is. If it's muscle building then you want to train that area the hardest and take it easier with the cardio. On the other hand a professional sports person is more likely to hit the cardio hard but not train weights so hard that it compromises performance in their sport.

So you've got the right idea by doing your cardio after your lifting if you want to prioritise lifting. And a calorie surplus will optimise the strength and size gains, but try and not go overboard with the surplus, or excessive fat gains will start affecting your swimming performance.

The priority is muscle building. With the swimming I won't be doing anything intense, I don't know how to swim so will be practicing a crawl each day for 30 minutes. Does that count as low intensity?

What exactly is an example of low intensity cardio? And can I maintain good cardiovascular health while doing it?

For the surplus have I got the right idea? I'm eating a 300kcal surplus for the resistance training and I''ve searched up the average kcal burnt during a 30m swim and planned on eating that. Is that not necessary?