The majority of homeowners insurance policies do not provide coverage for flood damage. Unless your insurance policy provides coverage specifically against floods, you would have to have flood insurance through FEMA or the NFIP.
The answer will depend on the coverage you currently have on your policy. There are two instances where boat insurance would cover an injury to a passenger caused from falling overboard. If your policy includes medical coverage, your passenger's medical bills should be covered up to the limit on your policy.
As long as you aren't driving the vehicle and it isn't registered then there should be no issue leaving the vehicle off of your policy. If you plan to drive the vehicle on a public roadway, you will need to insure it.
For most personal auto policies and motorcycle insurance, a permanent address is required to properly rate the policy. My advice would be to use the address where you receive mail for the purpose of obtaining insurance coverage.