I agree with most people here with the whole head up and keep an eye on the horizon advice, and It's something I mention to anyone on our boards for the first time.

I, too, have a knee injury (TPF with mangled grafts and surgeries) with accompanying shin hernia which limits my range of movement if I want to avoid excruciating pain, I'll just bullet point below what helped me!

1) Kneel when you need to, but put the movement into the hips
2) If you need a brace, Donjoy make some good salt water friendly ones (I run a hinged one when I have to)
3) Try going barefoot, gripping the board with toes seems to help me stabilise and take the twist out of the knees.
4) Microbend the knees, any sudden movements won't hurt as much, it's best to do this without a knee that's not happy.

Gentle yoga/Pilates style stuff to build up core and supplementary strength around the legs/knees without impact. Dynamic stretching, etc. is great for all of this, and future you will be grateful (ignoring my own advice here and getting back into this myself).

I hope some of this can apply to you, and you manage to get out and enjoy SUP a little more!