I know this is late, and you may not even see it, but I get alarmed when I see parents say "it's your decision, you'd know best."
In the case of water safety and infants, I'd definitely error on the side of extreme caution. You might even consider going through a water safety course or meet with a water patrol officer in your area. If you're not sure how to contact one, most water patrol units are officed out of your county sheriff's department. They will be very happy to help you choose the appropriate life jacket etc.
Until then, NEVER rely on a carseat combined with a floatation device or life jacket as has been suggested by others here. Many parents make the very tragic mistake in believing the "bucket" will somehow float if the child is wearing a floatation device - they will not float and are not designed to be floatation devices.
Keep in mind, even the life preservers designed for infants are not totally reliable. The life preservers (especially for young children) are designed to hold their heads above water and that's all. If the child is not properly secured in the device, there is still a risk of drowning. Many youngsters have been known to slip through or be held face down by the "jacket". With this said, it's pretty clear the weight of the car seat would be more weight than the life preserver could support.
Worse, if the child is strapped in the car seat, and something should happen, the child could be trapped and drowned before you could ever retrieve it(seat) from the water since its weight would cause it to sink quickly, and the life preserver could become an entanglement danger to the child.
Using the bucket seat could also create a false sense of security, and parents might be apt to leave the bucket seated on the floor unattended, and if for some unfortunate reason there was a boating accident (most are the fault of other rec boaters/vehicles in the area crashing into you, not the other way around) the child would be at risk for injury or worse. I guess what I'm saying here is that while there are dangers on your end to prepare for and be aware of, most tragic water related accidents involving children are the result of others (recklessly) boating and jetting in the area. In some cases many of those operating boats and jets skis aren't even skilled or experienced enough to handle these powerful machines, yet they do and in many cases under the influence of alcohol. And unfortunately there is nothing you can do to prevent that.. your only defense is "preparation" which goes only so far.
Think carefully. If you can't give your full attention to the child and still enjoy the outting, perhaps the previously shared advice of leaving the child with a sitter IS THE best advice.
As for myself, I say young children have no business being included in water activities until they can swim and fend for themselves. As infants they're too vulnerable. As toddlers they're too unpredictable, and as teens they're too unreliable. Your best defense is to get them swimming lessons, and as many safety chips on their side as possible before putting them in situations like that. And even then parents must use extreme caution and take appropriate safety measures...unfortunately because you can bet of all the others recreating that day, someone (and guess I'll it's more than less) out there is not exercising caution. Remember water related accidents are one of the leading causes of child mortality.