I agree with
@Kaldak
I ran track my whole life, but began playing waterpolo because I wanted to be in Navy SOF one day. I was the slowest swimmer on my team. I eventually got to the point where my endurance could handle the hours of swimming a day that the other guys who had swam their whole lives tolerated better than me.
One day my water polo team did a land based workout that included a lot of running and I noticed something interesting. These guys with 8-pack stomachs who could swim insanely fast for insane amounts of time were useless on land. They were indisputably in amazing shape, but it was comical to watch them run a single lap around the track. I gleefully lapped them over and over because I was finally better at everyone at something.
Your body adapts to what it is used doing. Just because I was a fast runner who could run for distance, didn’t also mean that when I first started swimming I didn’t embarrass myself like a drowning fat old woman. These guys who were amazing swimmers were exhausted running one slow little lap around the track.
Just keep doing it, my man. If your body is shocked going from swim to run, then keep practicing that transition. You will adapt.
If you are doing this for the Physical Screening Test(PST) then try and do the simulated Test as often as possible. Putting it all together often is a lot more difficult than doing it separately as you have seen.
I also would practice treading every day you can get to a pool, because treading takes a long time to get good at. I’ll post treading videos soon.