Difference between $50 and $175 breaker
The reason why Zinsco/compatible breakers are so gosh darn expensive is because Zinsco panels are obsolete and have a somewhat checkered field failure history -- while they do not suffer as badly from the issues that plague FPE's line of non-breakers, the Zinsco breaker line still has some of the same problems: namely miscalibration (i.e. failure to trip under low to moderate overload conditions) and breaker-to-busbar arcing/overheating damage.

As to a replacement type: the resistance of modern electrical grade aluminum alloy (AA-8000) to salt air corrosion (pitting) is unknown, but presumed to be poor -- also, salt air and current drive can aggravate galvanic corrosion of aluminum, weighing against the use of unplated aluminum busbars in salt air, and also against zinc-plated aluminum (the standard for aluminum busbar panels) due to bimetallic effects.

This leaves us with copper busbar panel options; some use various forms of plated busbars (either silver flashed or tin plated copper), while others use a bare copper busbar. While silvered contacts are generally considered to have excellent corrosion performance overall (tarnish is conductive enough in thin layers that it doesn't impact contact performance the way oxides do), the presence of chlorides (salt spray) can result in nonconductive silver chloride being present.  On the other hand, tin plating withstands salt attack very well in salt spray tests, and so does bare copper.

Another factor is that if the existing enclosure is appropriately weatherproof and in good condition, an Eaton retrofit kit can be installed by an electrician to convert the existing panel to a type CH (copper bus BR is not available in the retrofit kits).  Of course, if the enclosure's shot and needs replacing, then replace it.