Try floating plants. Water hyacinths and water lettuce float and grow and expand quickly.

We put ours inside floating hula hoops to keep them together and they form a nice island of shade.

We put an umbrella in our pond once last summer. The fish were so afraid of it, they avoided it, making its shade useless.

Pond lilies, water grasses, etc. are helpful especially if you live in an area with predators (as we do in Montana). I have friends with a long-lived koi pond and the grasses, lily pads, hyacinths, etc. are fantastic shade and protection when the need arises!

Nothing around my pond to tie a sunscreen to. Like the floating plant idea. Do your koi eat the floating plants?

Absolutely - consider the floating plants their "roughage" - keeps them healthy and strong.