I did my first bowthruster service this past winter and used a 29x42x8 seal. It was a tight fit but it works. The seal I replaced was also a 29mm ID seal. There was no oil leaking from the seal while the boat was sitting on the hard after the service and the oil is still clear - not the "cafe creme" looking mixture of oil and water. However, when I disassembled the unit to replace the bearings (preventatively) I could feel a small groove in the shaft where the previous seal sat. The shaft was also discolored with a brown-ish stain at that point:
I consulted with Olivier Beaute and he told me that if I felt any kind of groove that I should try to gently sand it out with 600 grit sandpaper and water. He also said that if I could NOT feel anything that I should NOT sand it but just clean it as sanding the shaft comes with it's own risks. I measured the shaft OD at the point in the shaft where the oil seal sits with calipers and found that it was about 29.5mm so there was plenty of "meat" on the shaft for a 29mm ID oil seal so I wet-sanded the shaft smooth, re-assembled everything and so far, so good. However, a 30mm ID seal would have likely resulting in a static oil leak like you have and certainly would have allowed water to enter the bowthruster during use. Consider taking the entire bowthruster shaft apart, measure the shaft OD and inspect it for any damage where the oil seal sits. Also check the OD for any damage as well.
SM #440 Cara
Deale, MD USA