Jason (the aptly named skipper) used the hydro-phone to help him find whales. It relies on whales' own tendency to use 'active' sonar' ie to emit noises in order to find things. (Passive sonar is just listening to other noises already happening.) It's a clever bit of kit, of a piece with a very professional operation.
Of course, the whale watching boats talk to each other about the whales they've found and what they're doing. Sperm whales dive for up to an hour and then spend five to fifteen minutes on the surface recovering. So if one is spotted, the others have that long to get to it. Also, the whales that are around Kaikoura a lot don't necessarily travel far during their hunting dive, so come up in roughly the same location where they were last seen.