I have a "Johnson Pump AB Sweden" F4B-902, 10-24509-02 raw water pump on my Yanmar 3GM30, E21106 engine. There is a small leak and I suspect a shaft seal. Since it has only done 150 hrs of work I doubt whether the whole pump needs replacing and anyway I'm sure that would cost many hundreds of dollars so I'm after a seal repair kit. I've not yet removed and dismantled the pump to confirm my suspicions. I'm just checking my options at the moment. After a couple of hours internet searching I've found nothing.
Is there some sort of seal repair kit for this available in Australia (or anywhere for that matter)?

EBay. Search water pump 3GM30. There are a couple of variables. I think the Johnson pump has an O ring rather than paper gasket. The other issue is that some of the pump shafts hold the impeller on with a woodruff key, and others have a cut through the shaft, so they have a threaded rod across the inner diameter of the impeller.
If you've got reasonable access, check that the leak is from a seal and not the impeller plate. It's definitely worthwhile getting to know your water pump and carrying a spare impeller at least. When pulling it apart, check for wear on the front plate as this is a common cause of poor pump output. Some front plates can be turned over once worn, others not.

These pumps are very simple and hence are easy to repair & recondition. If it's not the impeller cover leaking, then the drip will be coming from the seals via tell-tale hole in the bottom of the casing. Take the seals and bearings to the bearing shop and get the generic equivalents (for about 1/3 the price of the Yanmar branded ones). Make sure you tell them they're for use in salt water, and that the new seals have a stainless spring (check with a magnet), or get them to replace it (the spring) with an o-ring.

A seal leak almost certainly means the shaft has been scored a bit, so you'll need to clean it up with fine paper. Just focus on the bit where the seal lips sit, the rest will be ok. Start with something like 400 and go finer till you have a mirror finish.

Mine has been drip-free for about 5 years/500hours since I did it. You can buy Yanmar pump kits, but they are ridiculously expensive. If you DIY, the only Yanmar bit you need is the impeller.

Cheers, Graeme

Thanks for those helpful links Jolene. I've saved them.
I removed and dismantled the pump today. It was a leaky seal so will purchase another tomorrow. It is a standard seal it seems - 25mm OD and 12mm ID and will cost less than $10 from a local bearing supplier.
There was an O ring on the shaft but outside the seal between the bearing housing and pump housing and serving absolutely no purpose from what I can see. I managed to locate a parts diagram for this exact pump and the O ring is also shown sitting in the middle of nowhere. Anyone know why ?

The "O" ring is mostly likely what is known as a thrower ring to throw water off if it is leaking from the pump and prevent it from going to the bearing housing.

And make sure the shaft is nice and shiny under the seal

A small correction for future readers: the seal OD is 24mm, not 25mm.
Thanks for the explanation of the oddly placed O ring. That does seem like a good reason to have one.
I have replaced the lip compression spring with a rubber O ring - also suggested by the bearing supplier. Interestingly, the old spring in the leaky seal was not stainless steel but also not corroded.
The cover plate (held on by 6 machine screws) has a slight circular 3mm wide groove worn into it by the impellor central cylinder, where the base of the rubber flutes join. It's about 30mm diameter. I've yet to clean the cover plate up but the groove will no doubt remain. Is that going to cause a problem?
(Just cleaned it and can only see a little bit of light under a steel rule placed across it so it will be fine I believe.)