Customs will be more interested in whether you're trying to import a boat obtained in another country. In most cases such an imported boat would require payment of customs duty. You'll need to show that you owner the boat in the US before you left. Documentation is considered proof of ownership, but is not _just_ a proof of ownership. See Vessel Documentation Information & Resources - MarineTitle.com
for more info on documentation. See 19CFR4.94 (do a Google search) for info on requirements pleasure boats entering (or re-entering) the US.
Registration is one way of proving ownership, providing it is up to date. You can renew documentation (and registration, in most states) online, but hard copy is difficult to have sent to you overseas. In some cases we've been able to obtain an electronic copy of the documentation, which has sufficed so far. My advice: document your boat if it's eligible (it's cheaper, too), or if not, register it and keep the registration up to date. Keep a copy of either the registration or the documentation on board. Note that some foreign countries still want to look at the _original_ documentation certificate, with its certifying stamps.
Don't forget that you'll need to have your USBPS decal up to date if your boat is required to have one. See https://dtops.cbp.dhs.gov/