When water enters an engine it can cause a hydraulic lock, as the water isn't compressible. In that case, the engine won't turn over at all, as the pistons come against water in the cylinders.

It sounds more like an electrical problem. There are lots of electrical devices in the X5 that don't take well to being submerged, like fuel pumps and control units. The usual damage point is at around 20" IIRC. Recommend you get the codes read and see what errors are showing.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue