We keep our boat in the water all summer as well and I have had good success with the Poppy cover. The ratcheting mechanism is located right at the water line so it stays wet. I simply try to keep some oil on the parts a few times a year, something that sticks, like anti-corosion spray. When I do take the cover off I let that wet part dry out completely. So far after 8 years, have not had any issues with the material or the metal parts failing. The sunbrella material itself does tend to wear out over time and you can prolong the life by using 303 protectant every year. The sun & UV rays are worse for the material than being in the water. Think I may finally need a replacement this year as the fabric is getting a bit weak and breaking down, it will be another poppy though.
Hope that helps answer your question.