You can hook up all the batteries in series you want, but you're still limited by the voltage rating of the consumers if you don't want to add converters. A Tesla Powerwall battery pack seems to be operating at 50VDC so if you don't have any 48VDC or 50VDC appliances you will still need either a DC/DC or DC/AC converter.
When talking about small scale homesteading/seasteading converting between AC voltage levels is not relevant. So that advantage is also not relevant. The moment it becomes relevant you're not talking small scale any more and you are using a serious amount of power. The cable lengths you can expect on a small seastead are the same you can expect in a home, you're not trying to power something a mile away, so even for DC cable sizes would be ok if you don't have heavy consumers on DC.
Inverters you should always buy pure sine wave inverters, even thinking about using a modified square wave should be frowned upon.
Propane, butane, methane, LPG, gas, diesel you probably need to go out and buy it somewhere.
Solar, wind and wave can be done onsite and thus would have my preference.
On ships chilled and heated water are tied into the air conditioning system with often a resistive heater in the fan coil. Water in the tropics can't really be used to pump through some lines and use a fan to cool a room, the water temp is too high. Water in the arctics can probably be used for that purpose, but you probably have more need of heating in those areas.
I would recommend having an emergency generator present any way, and it can be your system philosophy to run a generator every time you need to use one of the heavier consumers.
Generating DC to make hydrogen, store it and convert it back to electricity or burn it is just a bad idea efficiency wise.
Marinea doesn't really count as small now does it?
A small seastead to me is basically a single homestead on a barge, maybe a couple of them together but all of them self sufficient energy wise.