Yes, it has plenty of power for a hot water heater. Yes, you can get an adapter "pigtail" - a 30amp to 20amp pigtail. 20 amp will use a "normal" household plug and then a short, sufficiently sized extension cord will work to connect the genny to the pigtail.
Portable genny's are nothing new to the boating world, but they need to be used with lots of caution - every year there are deaths associated with portable genny use - even when people thought they were using them properly. Using them on a boat is not the same as using them on land. Electrical grounding is different, gas safety requirements are different, even exhausting is different.
I'm copying and pasting this (below) from another website - partly because I'm lazy, and partly because I don't think I could say it any better. If you decide to use it, just be extra careful and treat this situation with the caution it deserves. This is not something where you just get it going and "forget about it". Be smart with this so we don't read about you in the paper!
1.Carbon Monoxide (CO) is present with all generators, but on portables, the exhaust is discharged at the generator, within an inch or so of the exhaust port on the cylinder. The exhaust system on a portable generator is usually constructed of non-marine alloys that can rust through after brief exposure to a salt water environment.ﰀ There is no engineered system to remove the exhaust from near occupied spaces. When placed on a swim platform, normal air flow can cause a station wagon effect and suck CO into the cockpit or cabin. While the same problem may exist with marine generators, the exhaust is mixed with cooling water and is discharged well away from occupied space at or near the waterline and is usually cleared by wind or sea breezes.
2. Fuel Systems on Honda's and other portables are vented to the atmosphere, not internally as with marine generator installations. That means explosive gasoline vapor is released at the generator, usually through a vent in the fuel fill cap. There is also the risk of a fuel spill if the generator is upset or you encounter rough seas or a large wake when the generator is close to full.
Their carburetors have a bowl drain that releases fuel inside the generator case. That means where you run it, store it or put it under way will have gasoline fumes released in the area and if the bowl drain leaks, you have raw fuel spilled.
Portable generators has simple fuel fittings and single ply fuel lines attached with hose clamps, both are substantial risks to fuel leakage if the generator isn’t new and has been around a while. Marine generators have USCG approved double ply braided fuel lines with swaged on end fittings that thread into the generator fuel pump.
3. Ignition Protection – None of the electrical components on portables are ignition protected. Marine generator electrical components are.
4. Shock Hazard Exposure –Portable generators pose an additional shock hazard since the portable is not grounded to the boat or to a shore side ground. Likely not a problem with a drill or power tool, but if you connect it to your boat's AC system, you have essentially disconnected the green wire. Yes, most portables sold today have 3-wire systems. Where the problem lies is that the portable generator is not part of the global ground when you plug in. When you plug into shore power you are also plugging into the ground for the entire local power system. When using a stationary mounted marine generator away from the dock, your boat is its own ground system. Even if the portable has GFI circuitry, they won’t work if there isn’t a continuous ground system. I think real risk here is that while the boat owner may understand grounding, not everyone on the boat….kids, wives, girl friends, helpful (?)guests…..usually don’t.
Additionally, most portable generators now use invertors. An inverter drives both line and neutral so it is possible to have voltage between neutral and ground. With ground bonded to the boat's bonding system, which mean to the water, this means a shock hazard may exist that normally should not.
