dk said: "The boat supply company I found on Google carried 20 and 30 gauge vinyls, so I assume both are pretty heavy duty. 30 would be the lighter of the two, so I might go for that."
The 30 gauge is the heavier of the two, and it's very heavy (30 mil). Plastic film gauges are the opposite of wire and sheet metal gauge. The larger the gauge value, the thicker the film. The 30 gauge PVC is 50% heavier than the 20 gauge.
To complicate things further, sellers and users of PVC film conventionally conflate gauge and mil. So, for PVC film, 30 gauge = 30 mil. This is very heavy film. Here is an example:
For other kinds of (non-PVC) plastic films, gauge usually equals 1/100 mil. So, if you bought some 100 gauge Kapton film, for example, or Tefzel or Mylar, you would find that the thickness was 0.001", or 1 mil (30 gauge Mylar is 0.3 mil or 0.0003", etc.). A piece of 30 gauge PVC film is one hundred times thicker than a piece of 30 gauge Mylar, in other words.
Someday the wisdom of the metric system will rescue us from this nonsense.