They were good engines but as Tom points out they are pushing 40 years. Lots of other engines too and they also are starting to have parts problems so the 555/903 are not the only ones.
My understanding is that the 903 was carried in production some years longer than the 555 and was used in the military so parts tend to be more available than the 555. However, for marine bits such as water cooled manifolds, heat exchangers that won't be the case.
Your best bet is to call the local Cummins dealer and distributor and ask that question. I know in my area I can still get any peripheral parts for my 555's. The PT injection pump I can still get rebuilt and parts for along with injectors.
The raw water pump may be a problem in the next few years but I have been looking about.
Many of the knowledgeable mechanics and service people are retiring or have done so. I look after most of my own needs. There should be people around though but you may need to ask around the marinas
If they start smartly from DEAD COLD and the smoke clears quickly or reasonably so then the internal condition is likely ok. Untill there is some load on them they may smoke as the combustion chamber is cold and won't really warm without a load.
If you do pursue the boat be aware there are some needs that are somewhat different from a lot of current pleasure engines. Not difficult to deal with but must be done. The biggest is the neccessity of using DCA chemical in the cooling system along with DIESEL rated low silicate antifreeze. This is the case with all Cummins LINERED engines and the 903 is one, as is the 555. Actually all linered engines need this or similar additives. There is a test strip kit to test and adjust the DCA level.
These engines require good batteries and wiring. There are no starting aids such as glow plugs or grid heaters so the cranking speed must be good or you could experience a no start even though there is nothing wrong with the engine. I had that once and have made very sure to never happen again.
And yes, go to Boatdiesel.com. There is some information available there including the setting specs and adjustment procedure for injectors and valves. There is an owners manual which covers several engine models including the 903. Look for it under the 555 engine. The manual is in 5 parts. That is how I posted it. You will need to become a member though to read and download it and search older threads. Do a search under my name, clark leighs, going back a bunch of years for some threads about this engine. Search for both 555 & 903 as a lot of mtce info is applicable to both.