I don't know if this would be a helpful approach, but if water depth is going to reduce pump performance by too much then you might think about an "air lift" pump rather than just a diffuser alone. Injecting air at reasonable depth into a vertical pipe would allow you to place an "intake" deeper in the pond and only have to pump air down a few feet. I haven't seen a discussion of that method on PB and I've only seen it in a tanks system, so I'd be curious if anybody has any experience/opinions around as it applies to a pond setting.
Here are two diagrams. One is open ended and the other draws water through a gravel-filter intake.
http://www.ca.uky.edu/wkrec/AirliftPumps.pdfhttp://toddecological.com/PDFs/100623.casestudy.fourseasons.pdf
