What paint should I use on my aluminum boat?

No it is not too expensive, it is just the price of good quality coatings designed to do the job. If you have removed all the original coatings you can do one of two things - leave it, as the natural oxidisation of the raw surface actually protects the metal as well as any other coating thus far tried. Or you need to use an aggressive etch primer, followed by a base undercoat layer then at least two finish coats. Normally these are (poly)urethane and generally two pack so not to be used at all unless you are taking all due care as all two pack finishes are very toxic. Yes you can use basic enamel over a quality etch prime - but the coating will not suffer abrasion or scraping over rocks etc. well at all. With etch prime the point is to get a fine constant mist coat over all areas - it is not a paint just a boundary layer and you should be able to see through it in most applications. The undercoat layer should be opaque. As the boat is over 50 years old I suggest just leaving it natural and get some use out of it.

If there is paint on it now you have to sand it completely. Use an oil based primer and allow about 7 or 8 hours to dry. Use your choice of color in water resistant paint. Allow another 8 hrs to dry And apply a clear coat to protect against the elements and scratching. Go Fish!

It's a $300 fishing boat. Why spend a lot of money on paint? Just leave it the way it is!

No paint because paint hides corrosion and that's bad news.