Yoadrian,
Do a search on Black Box, there have been several good posts recently on the topic. Some boats have a natural voltage that is already near ideal for salmon fishing and some don't. Downriggin' certainly catches his fair share of fish and swears by the BB. I personally think my BB has increased the number of bites I get by somewhere between 30%-40%, but I am still testing mine out. Even if it doesn't do squat, after spending $5K on a boat and committing endless amounts of time chasing salmon in the salt, a $100 investment is pretty darn cheap insurance that you're boat is attracting and not repelling fish...just my $0.02 worth.
JMS