a bomb at the bottom of the water in Britain (given the historic context of Germany bombing) has probably detonator in the armed state. The detonator just didn't triggered back then because for example the bomb hit the mud (naturally for the bomb falling into the river). That detonator still has high chances of going off in any moment as result of rust and/or any disturbance like from a boat passing too close. So the earlier such bomb is found [by anybody] and is removed by professionals the better. (back then, during the 4-6th grades, we found and "responsibly" disposed off [in a campfire in a forest outside the city] a range of WWII munitions. All without detonators of course. The detonator makes huge difference and we wouldn't touch the stuff with it attached - didn't even attempt to check the state of the detonator, after all we were after the fun, not suicide)