ABS is stable after some humidity is absorbed. plastic models are made of it and I have used them under water in aquariums for years. same for wood it takes a hundred years for wood to break down in Ocean water even

The lack of saltwater termites helps.

If you want to print thicker lines, you can up your extrusion multiplier. You'll over-extrude a little bit and it might smoosh things together. After doing that, you can use some acetone (either painted on or as a vapor bath) to seal the model together a little more. This is assuming you are using ABS.

If you're ever curious about what can and can't hurt ABS, just remember that it's the same plastic that LEGOs are made of. If it can hurt a LEGO, it can hurt your model.

Hey guys, I am trying to make underwater housing so that will required no gaps if possible.  What settings can I adjust to make the infill more solid?  Any suggestions?

It is impossible to print a water tight model without using some sort of sealer. Use also would have to use ABS as PLA will degrade in water over time.