Are the spiders coming aboard while you are actively using the boat, or do they move in while it is unattended?
I'd suggest using one of the herbal insect repellants that are made essential oils, like citrus, eucalyptus, peppermint etc. You might need to sample several different brands to find one with a smell that *you* can tolerate.
I have heard from some homesteaders of my acquaintance that the Osage orange fruit (aka Hedge apples) are a natural spider repellant. They place the fruits in the corners of the house and garage and then throw them out and replace them once they've dried out.
Have you considered bringing along one of those battery
powered hand vacuums that have a long extension tube just in case?