A couple of things come to mind that these engines are known for. The oil filler warps and can allow water to enter the crankcase. See item 15 in the link below. It cannot be simply resealed by replacing the gasket but must be replaced with a new filler AND gasket (item 16 in link).
This requires that the powerhead be removed to do the swap so anytime major engine work is done the filler neck should be replaced as a precaution. It's one of the few weaknesses of these stout little twins.
https://www.boats.net/catalog/honda/outboard-by-hp-serial-range/8hp/bf8am-la-1997/oil-pump
Take a look at item 2 in the link below. That is the cylinder block to oil case (sump) gasket. There is a small oring "nestled" within that gasket. You can make out where it is placed in the depiction by noting that there are 8 small bolt holes in the gasket and the one slightly larger hole in the upper corner is the cutout for the oring. The passage that oring seals is where water coming from the pump, up through the water delivery tube, enters the engine block under force. If that oring doesn't seal, water will enter the oil sump. I'm guessing that, if you replaced the sump gasket that you probably replaced that oring???
https://www.boats.net/catalog/honda/outboard-by-hp-serial-range/8hp/bf8am-la-1997/cylinder
The sad news is that, as these engines age, parts support lessens a little every year. They didn't even bother to post a depiction of the oil case for your model. But, I can provide a link that will have a blow up of the oil case from another model since they are all built to the same "platform". The part number for that small oring will be with the oil case. Let me know if you need the link.
Other than those areas it's possible that the block or the cylinder head could have a crack that you didn't detect when you had it apart. Hopefully that's not the case.
I'm sure there are other possibilities that I'm unaware of but we don't know what we don't know.
Good luck.
