any weight lifting (unless youre a body builder) should be low weight with many reps.

Your will build stamina and strength while not building bulky muscle.

You also have less chance of hurting yourself and over developing muscles.

i think for sup, the rep speed is important too. low weight fast high reps

Depends what you are training for, endurance or short sprint races. For short sprint races you need to get off to a flying start so you need your muscles to have explosive power and to use your short twitch muscle fibers and to teach your body to work efficiently in an anaerobic state. Conversely training for longer races you need to call on your slow twitch muscle fibers.

Thats if you are training for a specific event, however for general fitness you need to find the balance.

All the above
Mix it up

High reps 20 up to 50 or more
low reps 3/4 up to 10 or 12

Laird Hamilton's book "Force of Nature" has a chapter devoted to gym work specifically for SUP. He calls it "The Circuit" and it incorporates 15 exercises. He suggests three ways to complete the circuit:
1. Basic - 3 rounds - first is 25 reps; second is 15 reps and third is 5 reps. Weight is altered so that you can just complete the targeted number of reps.
2. One-round Endurance - 40 to 60 reps of each exercise, maintained good form (with light weights).
3. "The Grand" - 6 rounds - 30 reps; 20 reps; 10 reps; 5 reps; 15 reps; 25 reps.

Circuits 1 and 3 take lots of time 1-2 hours...so for us mere mortals, that is unlikely to be a common occurrance. However, I have altered the Basic Circuit so that it can be done in under an hour.

Or, you could just SUP more and in doing so, will develop the stamina and muscle tone required for SUPing!

It's much more interesting too than looking at yourself in a mirror at a gym!

High reps of bicep curls every night with light weight....what does a stubby weigh??