A hydrophone was not useful at full speed

Could a Dog Boat's hydrophone operator work effectively while the engines were running at full speed?

Not comfortably or usefully. Engine noise would be severe, especially aboard a boat fitted with Griffon engines.

The Fairmile D's sensor list includes a Type 715A hydrophone for anti-submarine work. Full-speed engine noise would make listening extremely difficult, especially aboard a Griffon-powered craft. The problem is a practical human-factors caveat. Installed equipment creates a useful capability only under operating conditions in which the crew can use it.