First of all, confirm IF it is indeed sago pondweed. Battling a weed problem without first positively identifying the culprit is often a losing proposition.
"Chemicals" (herbicides) are simply another tool in the box for managing unwanted vegetation, and like most tools, each type of herbicide possesses attributes or drawbacks for varying scenarios.
In other words, many more details about your pond are needed before considering which (if any) herbicide will serve your objective. It's somewhat like asking what caliber is best for deer hunting. Lots of variables to consider.
Pond size? Depth (max & overall avg)? Water-uses (irrigation, livestock, etc)? Flow-through or static water? If water transitions through the pond, how often and to what degree does the water transition? What is your objective? Partial or total control? Duration? Are any desirable plants currently present (if so, identify them)? Do you have (own or access to) any type of application equipment - or knowledge of its use.
Those are the basic questions for determining if or which herbicide might suit your circumstance.
