It is not so easy for the water to get inside the frame, and fortunately in this case, the oposite is also true: it is easy to keep it out. You can use hot glue to cover a bunch of tiny holes meant to let fumes out during the welding of the frame. These holes are usually near the extremes of the seat stays or chainstays. Hot glue won't stick too hard and wont damage paint. Can be removed by just the fingernails.
The other tubes do not require these holes as they connect to some other. Top Tube for example has ventilation trough a hole in the seat tube (remove the seat post to check if it's there) and the headtube. The same for the downtube, it comunicates to the headtube and the bottom bracket shell. This limmits the water entrance posibilities to the bb bearing, the headset bearing and the seat tube collar.
Good bearings correctly installed should have seals that avoid water getting inside them, and also keep it from geting into the frame.
The lower end of the steer tube is most of the time open, you can seal it by using a cork or a similar rubber snap-in seal...
For the headset bearings there are comercially vailable neoprene or rubber sleeves, but you can make do with ol inner tube of an apropriate size that fits tight (For the top bearing: just remove the stem, slide the rubber and replace stem.)
Similar thing can be done for the seatpost collar, the short cut usually towards the back of the bike collects water and debris thrown by the rear wheel. It can be protected with either commercially available neoprene sleeves or with old inner tube. If you can't find a tube that fits tight you can lessen it's diameter by cutting it and gluing with vulcanizing compound (a.k.a. patch glue).
Some frames have unused screw holes for accesories like water bottle or the like. Cover these with correct boltls or hot glue. There are also plastic plugs for this purpose (if you can get hands on them), some of wich resemble bolts.