Water pumps on V8 E39 die. They die repeatedly and predictably. Whether the impeller is plastic (then it disintegrates and contaminates the whole cooling system with debris - but most of these are already dead), or metal (then you're subject to bearing seal leakage).
Mine did too, at 62k miles.
Having learned the lesson the hard way, I've decided, with the help of my friends, to replace it myself.
Learned quite a few things.
If you're thinking whether you need to replace the water pump or not, you're a fool. I was a fool. It will fail. I've heard an opinion once that BMWs must be maintained right on schedule, not a second later, or they fail catastrophically. Respect this rule.
You can't identify the leak source without performing a leak test. Even if it looks obvious, the leak may not be where you think it is. Learning this experience cost me the price of parts 11537505229 and 13621433077 combined, at dealership price.
A simple contraption of a piece of soft rubber or plastic pipe, plus the ball adapter for a bicycle pump, is a decent substitute for the BMW special tool for performing a compression test.
You have to manufacture a special tool to get the fan assembly off. Instructions are here. Heed the part where it says it has to be steel - aluminium will not hold the torque necessary. It might, once, if you're lucky. But you're probably in a crunch to get it done, so don't do it unless you have a piece laying around. Oh, and anything thicker than 1/8" will not fit.
If you replace part 11537505229 (lower water hose), you must replace part 13621433077 (temperature sensor) as well. Old sensor put in a new hose will leak, even if it is one year old. It's all about O-rings - actually, one could just replace those, but I'll be damned if BMW sells me one, and I'll be twice damned if I dare to risk the cost and nuisance of getting another leak and getting inside the car again because I cheaped out on hoses. Wasn't that bad, actually - I found the leak before the belly pan was installed, so all it took was to drain the coolant from the radiator (not the engine), and replace the sensor reaching from underneath.
Your heater control assembly (vicinity of part 64118374994) will leak, too. It is possible, though, for it to leak when performing a leak test, but not normal operating conditions. I'm keeping my fingers crossed.
Draining the coolant is a wet and dirty job. Get yourself a pair of those chemical googles, sorry, goggles, and a decent drain pan, it'll splash a lot. Five gallons is good enough.
You have to drain the engine block to replace the water pump (unlike water hose replacement, when it is sufficient to drain just the radiator).
It is a very, very good time to inspect the belt drive system and replace the belts, pulleys and hydraulic tensioners.
Same for hoses - unless they've been replaced already, they're pretty much dead by the time you have to replace the water pump. No big deal, especially on later models with quick connect fittings.
Torque wrench is a must, there is a couple of places where the torque is quite nontrivial - for example, the bolts fastening the pulley on the water pump shaft are literally finger tight. Screw that up - and you'll need a new water pump (somewhere between $150 and $300, depending on urgency).
Getting the water pump out is possible without getting the secondary air pipe part 11721433344 out. Tricky, but possible.
Getting the water pump seal in place is a bitch. I don't know if it is possible to do alone, but having two people do it makes it easy enough.
All in all, it was fun :) Unlikely that I will have to face this issue again in next five years, but if I do, I'll do it again.
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