BFR FLT, THE PLT OBTAINED FUEL SAMPLES THRU THE REMOTELY OPERATED GASCOLATOR, BUT DETECTED NO WATER. NO SAMPLES COULD BE DRAWN DIRECTLY FM THE WING OR FUSELAGE TANKS SINCE THEY HAD NO QUICK DRAINS INSTALLED. A PWR LOSS OCCURRED DRG TAKEOFF AT 50 TO 100 FT AGL. THE PLT SWITCHED FUEL TANKS (FROM L TO R MAIN) & ENGAGED THE BOOST PUMP. ENG PWR RETURNED AS THE ACFT STRUCK UTILITY LINES NEAR THE END OF THE RWY. THE ACFT THEN CRASHED & HIT A FENCE BFR COMING TO REST. A SIMILAR PWR LOSS OCCURRED APRX 3 WKS EARLIER. MAINT PSNL FOUND A LOOSE L TANK FUEL CAP WHICH COULD HAVE ALLOWED WATER TO LEAK INTO THIS TANK. A MECHANIC ADJUSTED THE FUEL CAP FOR A TIGHT FIT. THE ACFT WAS THEN FLOWN TWICE, PRIOR TO THE ACDNT, WITH NO EVIDENCE OF WATER IN THE FUEL OR OTHER DISCREPANCIES. HOWEVER, AFTER THE ACDNT, DIRECT ACCESS TO THE L MAIN TANK REVEALED THE PRESENCE OF WATER WHICH WAS COLORED PINK. PINK STAINS WERE ALSO PRESENT ON THE TANK WALL ABOVE THE AFT FUEL OUTLET PORT. THE OUTLET PORTS OF THE INTERCONNECTED MAIN TANK CELLS WERE 3/8 TO 1/2 INCH ABOVE THE TANK BOTTOMS.