Here is a potential problem with the Sherwood G-908 (or Westerbeke 42175) and possibly the Oberdorfer N202M-908 as well;
I have a Universal (Westerbeke) M50B with the Sherwood G-908 pump. Fortunately I was at the dock when the Sherwood simply quit pumping. I recently replaced the impeller (12-15 engine hrs ago) but I opened the pump to check if somehow the impeller was at fault but it appeared fine. It was at that point I 'bumped' the engine and found the impeller was not turning. I removed the pump housing and found the internal drive shaft was rotating just fine.
A Universal/Westerbeke mechanic informed me it was a problem he has repaired a number of times. Apparently the 'Tab A' on the Sherwood pump only engages 'Slot B' on the internal drive shaft MAYBE by about 1 mm. After a while the end of Tab A becomes worn and simply does not engage the drive shaft anymore (in my case <1000hrs). His solution is to have a welder build up the length of Tab A (or the face of the internal drive shaft, ideally both) another couple mm such that it extends deeper into the slot of the drive shaft. Additionally, it may be possible to mill the face of the pump a tad (so the shaft extends deeper) without compromising the structural integrity of the pump housing. Every teeny bit gained extending the pump shaft tab into the slot helps.
Note although the mechanic told me of this problem before he even knew what engine I had, this may only apply to a Universal/Westerbeke M50B. I am considering replacing the Sherwood pump with an Oberdorfer N202M-908 which is the recommended 'drop in replacement' (per mounting requirements). However, if it is an 'exact' replacement I'm concerned the length the Oberdorfer pump shaft tab is the same as the Sherwood. Comparing dimensional line drawings of the Oberdorfer with Sherwood dimensions I find the Oberdorfer shaft length is .55 while the Sherwood looks to be .518 (as best I can measure it with a micrometer). While better, I'm not entirely convinced it is enough.