I live less than 1/10th of a kilometer from a very large river, about 1/4 mile wide.(The Columbia River, sometimes referred to as the "Mississippi of the west") Vancouver lake is also a very large lake and used to be part of the river about 13,000 years ago. It's now fed by small streams and creeks. Lot's of streams and creeks flowing across the landscape flowing into both bodies of water.

On the contrary to what you said, I am never more than 5-20 minutes away from a large body of water. That's not the point. The point is, there are not any marinas on any of them unless I leave the state. Bottom line is, it's not very convenient at all to find a "boat repair" business near me that has that stuff on the shelf. It would be a major pain in the... And would probably cost more in gasoline than it's worth in savings. Still, there are other options. Many "home improvement" supply stores immediately nearby might carry such products. They just wouldn't be marketed as "boat repair" supplies.

For example, coconut fiber substrate for reptile enclosures. If sold as such, the price is very high. If sold as a garden supply product at a garden nursery, the price is much lower!