Well observed, they are rather expensive for not a lot of productiveness and something which is prone to leakage far to regularly.
A lot of folk simply use a tube of sorts with an angled and shortened prop shaft, which is effective but it makes some boats skip around, which is not only unrealistic, but again is none productive. So, for me personally, and everyone will have a different opinion of course, something with actual props would be nice, which when combined with the right type of motor would provide just the right amount of thrust for the size of boat it is in. Oh, and relatively cost effective - of course