"I'd be punished by the search engines, because internet marketing is not related to fishing."
I see lots of sites with varying topics on a single domain that do okay in the search engines. However, as a rule of thumb, if your fishing website is more than a few pages dedicated to fishing, it is smart to buy a more generic domain to advertise other programs with. This isn't neccessarily from a search engine standpoint, but more of a topical standpoint. Your will need to have links pointed to your subpages - if you add a subpage to your fishing site and it is not linked to from within your fishing site (which would look weird to visitors) and is not linked to from outside sites, it will have an extreme uphill battle ranking in the SE's.
From a PPC standpoint only it doesn't matter one bit. If the page is on a domain and not connected to any other pages and your sole purpose is to use the page for PPC advertising, you are good to go.
However, my suggestion would be that you leave your fishing site alone and buy a second domain and hosting (which you can do for under 50 bucks for the year) and as you said, go with a more generic name like internetdeals.com (I am SURE this is taken, was just trying to get across the point of generic) and use this site to market all of your PPC campaigns.
But, that would just be my preference ... with PPC advertising, you don't actually NEED to do this. However, keep in mind that with some PPC's like Adwords, your domain will have to be listed in the ad. So, if you are advertising digital cameras and the ad shows
www.fishingstuff.com as the URL, it may bring down your click thru rate.
However, this is all really just speculation and theory, as I usually keep everything seperate myself. I do have one generic domain for PPC that has some unrelated stuff on it - but the domain is very generic, like the example I used above.
LOL, ok, I am babbling way too much here today.