I think the electronics are the easy bit, but a tide line in the lens might show up if the water gets in the lens. Similarly water in the viewfinder or shutter box would be a problem, high speed burst might splash it around a bit!
Obviously the lenses would have to be similarly sealed if you wanted to actually use it underwater. Either way, just ensuring that the electronics won't corrode if your camera falls in the pool is the difference between a $600 repair and (at worst) a $100–200 cleaning bill. And if you design it with drain tubes so that if water does get in the wrong places, it can drain into the lens mounting area in front of the mirror box, you could typically clean it yourself by flooding the camera with isopropyl alcohol, draining it, and letting it evaporate. (Just use silicone seals; rubber seals would degrade in isopropanol.)
Please also bear in mind that your $5 watch has no user interchangeable parts that make a seal and maintaining the seal guarantee requires battery replacement by an authorised service agent costing $4 so you bin it and buy another!
There are two main reasons for those requirements:
1. The battery compartment isn't separately sealed, and the main seal isn't designed to stay in place when you remove the back. This is a cost-cutting measure in a watch that's really cheap and doesn't require you to change batteries every day....
2. Rubber seals had to be replaced periodically. With modern silicone gaskets, the operational life can be as long as 50 years, so that shouldn't be much of an issue unless you tear or dislodge the seal.
Both are readily avoidable through proper design.