Is it safe to go swimming if I have an open diabetic ulcer?

No. Not good for you or others in the pool. Risk of infection.
Swimming. No it is not safe for you nor, if this is a public pool, safe for anyone else. In addition, with most scenarios that i can think of, you would be without offloading and foot protection going to and from the pool. Discuss this issue with your wound specialist or podiatrist.