It would all depend on how far away you are from them. If you were within 4', you might get a little fill from it. Doing a fishing show, you're rarely that close, thus the only thing Frezzi is doing is burning battery. But you did say HMI - so is this a toplight or are you using a 200w HMI? If you're using the HMI, then you could have a usable range of 15' (or more if it's spotted).
Two suggestions: Take the light out with the other shooter and test it's usable range and practicality in a similar situation. Or ask the talent (and I use that word lightly!) to tilt their hat back a bit. The face will still be shadowed but it will allow more ambient light to fill the face.
Remember that the water will reflect the light to give them a fill. Your best bet is view the footage and decide if you need more fill. If so, then the practicality of an HMI and how natural the light looks will be your deciding factor.
These shows are generally understood to be outside in natural conditions and will be accepted by the viewers as such. They don't care so much about the fishermen. They just want to see fish. Although, I would definitely use an HMI for any stand-ups.