Spiders are a fact of life in a freshwater marina. After 40 years of Great Lakes boating and a decade living on a lake shore, it is most important to not fall behind in spider control. Start spider control before launch and don't let them get a toe hold. Once the first generation lays eggs, the population will explode.
There are environmentally safer insecticides, I use
Terro brand products (their ant killer also works well). Spray the inside of the boat and pay particular attention to nooks and crannies where spiders like to hide during the day. Typically I'll do this a couple of times before launch at the end of the day before I leave. Likewise get spray the areas where you find webs on the outside. They love to hide under sail covers, in the hems of Bimini's and dodgers, under the edge of Genoa tracks and numerous other areas where they can hide during the day.
Spiders also have a penchant for the rigging, especially near the mast head. There are lots of places for them to anchor their webs and they can hide in the mast head. Unless you're willing to climb the mast on a regular basis, these lucky spiders get a pass and we get to see gossamer threads of spider silk flowing from the mast head as we sail.
A less toxic method of spider control is a vacuum cleaner. Suck them up and their webs. At dusk they will expose themselves as they drop down from their hiding places, take this opportunity to suck them up or bathe them in Terro.
You will not win this battle until the first frost, at which point the sailing season is about over. The goal is population control, not eradication. As part of the process of preparing the boat for a weekend stay walking around the deck clearing the webs with a duster will become part of the routine. However with good population control, you will have fewer encounters with gossamer threads sticking to your face.
Disclaimer: As a rule we do not use insecticides, herbicides, and similar chemicals around the house or boat. However, there are very limited exceptions, like spiders on the boat. There are plenty of excellent spider habitats just yards from Second Star and which are much safer for the spiders and their beloved off spring. I like to encourage them to move to those more desirable areas.
There is one other method of spider annihilation that I can not recommend around boats. Discovered by a group of young men many years ago when moving into a lakeside dorm. A dorm which might have been best described as a spider megalopolis. The method? An aerosol can flame thrower. Quick, effective, and clean the incinerated corpses were vanquished. Spray deodorants, like Right Guard, used a flammable gas as a propellant, sprayed over a lit cigarette lighter the flame engulfed the intruders repelling the invasion.
Good Luck!