
Don't forget to maintain good flexability with plenty of stetching before and after exercise, especially before going in the water and when you have come out. It stops cramps & nasty injuries such as twists & sprains especially on wipeouts.
Staying strong but flexable is the key.
You will also need to increase your endurance for the paddleouts, especially in the heavier stuff. I found this out a few years ago on a strong swell which was a bit mushy a few years ago at Fistral one September.
Not going to teach you to suck eggs but to improved body strength do things like dips, press ups, squats etc. Using your own body weight as resistance. A good gym assistant/trainer will know these which you can find out and be shown for free at most gyms these days - as of course you may already.
As said swimming is excellent but also other things, including tai chi can help with balance, flexability & strength too.
All the best . . .