The exhaust backpressure is supposed to be between 1.0 and 1.5 p.s.i. I'm not sure about hte water presure, but I'd think it wouldn't be that great. Try the J.B. Weld, along with replacing the hot section of your exhaust. You can pick up the black iron pipe for this at Home Depot for about $25. If the J.B. Weld doesn't work, you'll be buying a new exhaust manifold from Moyer for about $350. All the attendant parts, such as the exhaust flange and heat wrap, will run you about $450 with shipping.
If it is an external crack only. It can be brazed, but of course the best thing is to replace it. My friend brazed his exhaust manifol when the flanged area broke off on a chevy 350cid. It held for about 6 years I know of......i2f
There are no visible cracks on the inside, so I'll try to get it welded. I was concerned that if the weld didn't hold if it would be dangerous if it blows. Sounds like the worst that will happen is that I'll get a soggy bilge... and a stalled engine if the water gets into the cylinders.
Take a bicycle pump with a built in pressure guage (PeP Boys $15.00) two tire stems with valves. Remove the hose from the barb at the thermostat housing and insert the bulb end of one tire stem and clamp. Remove the hose from the water injector on the exhaust, stuff the bulb end of the second stem in that hose and clamp it. Attach the bicycle pump an pump until the guage shows 20 lbs. (6 lbs more than the standard water pressure) watch the guage for 45 minutes. Should hold at 20 for at least 45 min. If not, you'll know before you go through a tear down.