What makes you think a 3000W model = 2 x 1500W models?
Bear in mind that many inverter ratings are in Chinese Watts (I'm not suggesting that applies to the one shown), so don't rely on 3kW-US actually being output reliably, continuously, in all weathers by all brands .
Dodgy practise to use inverters rated at (or close to) the needed wattage - far safer to include a margin for 'error' and increase the wattage by 20-50% therefore for a 3000W load you should look to use a MINIMUM of 3600W (next one up is probably a 5000W unit which would be ideal).
When you get to those kinds of power levels you might find they need a 24V or even 48V supply. Either way the power feed cables are 'huge' and any looseness or corrosion on the connections means lots of losses, lots of heat and plenty of trouble.
'Victron' make excellent charger/inverters and they can be arranged for parallel operation. Quality stuff but with the associated cost but if you mean to do it 'right' you can't go far wrong!
No they aren't just 2 x 1500W inverters, but it looks like that one rounds up. They spec 24.24A @ 110VAC which is 2668W, not even considering what both Alec adn Kelly mentioned, to pick one rated higher than the load.
You should know how many amps your motor needs, not starting the other way around. That inverter looks like it has 15A outlets so there's a 1650W per outlet limit right there, but you could use two outlets.
If the boat already had a 120VAC motor, why is there not already a 120VAC source? It seems some info is missing. If it had a working inverter previously that failed, why do guesswork instead of just buying another one or repairing it?
I think dave9 was referring to the fact that the boat already has the winch (120VAC) so must have some source of supply to power it??? Are you looking to replace a duff one or to fit one that can power it if the boat didn't already have one?
I'm surprised to see a 120VAC winch - even at 3000W the majority I've come across are 12V or 24V - far more convenient for a boat.
If it was an OEM fitting then surely - as dave9 infers - there would have been some AC source to power it?
I see a wheel there that you could turn by hand... otherwise I think I'd ask in a boating forum where there might be someone who has done what you're wanting to and can share their experiences with whatever they did, or see if you can buy one from someplace off their website but that has local stores with a generous return policy like walmart, OR surely a generator can be had for less than $10K? Are there special legal requirements for one on a boat, because I see 3000W or higher generators all the time for well under $2K, let alone $10K.
Kiwi is in the right area - the manufacturers may have a DC-equivalent system that you can swap the motor with however this would require substantial rewiring and control systems. I can see why the OP is asking the questions.
Gensets for marine use are no different (really) than any other but you have to plumb them in (air, exhaust, water etc) and space is limited in most cases so there are issues in fitting one.
The best solution is as I've mentioned above - a quality inverter. The aforementioned Victron range can be purchased in smaller sizes and outputs paralleled for occasional use at higher ratings so that you aren't using a larger-than-necessary system for everyday purposes.
Whilst fitting a cheap'n'cheerful inverter will (probably) do the job you're talking MARINE situations here and if anything goes wrong you're potentially a long way from assistance so cost-cutting is to be avoided for critical systems. OK, for an anchor chain you could just 'cut it' and leave it behind if the winch won't lift it! but doing that once negates any savings your cheap system made you in the fIrst place!