Yes, I would suggest a 300mm lens for distant shots of ships, such as in the shipping lanes, from shore on major rivers or seaports where the ships are 400,500,600 meters distant. The issue is a 300mm can be too long a lens, too narrow a field of view for bow-to-stern, side-view pictures unless the ships are so far away that most detail is lost. In my area, the Columbia River, we have many ships, and a 70-200mm telephoto is a good suggestion.
The mid-1970's to mid-1980's were the heyday of manual focusing, third-party zoom lenses, like 70-200 or 70-210mm f/3.5 lenses, or chaper ones, like 70-210 f/3.8 type lenses, or a few oddballs with 70-250 or whatnot....these lenses are now OLD, and available for $10-$35 used from pawn shops,
KEH.com, etc.
Asanuma, Quantaray, other oddball brands, MANY to chosoe from. I suggest
KEH.com because they buy only the better items, of each type...but a prime, 300mm f/4.5 or f/4 telephoto will be harder to find in Canon FD type mount....but they do exist.
ONE BRAND to look into would be Tamron Adaptall or Tamron Adaptall-II manual focus lenses, with a Canon adapter being the ONLY difference in a lens that can/could be fitted to a Pentax or Nikon,etc, or a Canon. Tamron made some nice Adaptall-II lenses: a NICE 180mm f/3.5 or f/2.8....I shot one, good. Same with their 300/2.8 and their 400mm f/3.5 super-teles. They also had some zooms...maybe a 90-300mm f/3.8???
I would say, also do NOT discount a 1970's-1980's era 135mm f/2.,8 lens: THIS is an EASY lens type to build well,and sharp, and cost is usually a mere $10,$15,$20!!! GOOD lenses, the 135/2.8 lenses of the 70's and 80's.