Downsides to the idea
1. Lots of hours on the main
engine which will wear out sooner and cost a lot more to replace than a
generator.
2. Also, not good to run your
diesel at low
rpm with a light load for long periods.
3. Most boats, running the main engine is pretty noisy as well.
4. The
alternator would not be able to supply the power needed so would slowly deplete your
batteries.
5. To get an alternator big enough to supply the load would cost a lot of
money, especially if you get one rated
continuous duty to put out a couple hundred amps. And it would take a serious mounting bracket AND unless you did something to counteract (serpentine belt, opposing idler pulley, etc) you will be putting a large side load on the crankshaft that could cause early death to the front
seals and maybe even bearings.
As a very occasional way to run an AC at
anchor it could be an option but wouldn't do it on a regular basis.