People jumping in the water would not help as if the person is asymptomatic they have the virus in them and will breath it out and be touching stuff. The UV light (sun) will help but if you have some hand sanitizer in the boat that might be the best I think as most likely transmission is hand to face outside. You could also just sanitize high touch areas on the boat, steering wheel if many drivers, glove boat etc.
Having said all of this is you spend time in close proximity time with someone who has the virus there is a chance for spread. Way less than indoors I would think but still a chance so something to consider of you are in contact with older people or your lake kids, ski buddies are also.