Top 5 places not to ride a motorcycle in the rain

Water
Well, sounds obvious, but it isn't. Never ride through a place covered with water if cannot see the bottom. Or at least, don't be the first. There are two ways around it. I prefer remembering what's below the water in detail, and then re-confirming that when someone else goes through it. The second, if you don't know is to look for a way around, until someone else takes it through the water and then following his wheeltracks exactly.

Manhole covers
Concrete one's aren't so bad, but if the concrete block has a metal band around it or is round, you could be in serious trouble. Metal has no grip whatsoever in the wet. Your only solution is to treat it like you were hydroplaning (which almost totally describes your predicament). Hold on firmly but no death grip please and wait for the metal to pass under. Two particular dangers are one, running half-on/half-off when the metal runs more or less in your direction of travel. Usually, this is a transient, slippery moment but most often it will cause no harm. The other is round manhole covers. You want to hit the metal at ninety degrees. The more perpendicular the intersection of the manhole circumference and you direction of travel is, the less trouble you will have. Speed helps a lot. If you aren't perpendicular, the manhole will catch your wheel (usually rear) and drag it sideways. Uncomfortable moment, can lead to crash.

Painted surfaces
On their own, they aren't so bad. But in the wet, the levels of traction on painted surfaces are much lower than whatever is surrounding it (concrete or tarmac), that can cause problems. You could be just fine until you start spinning up after hitting a painted patch. So pass between the painted stretches on zebra crossings, lane markings and be very cautious of crossing painted continuous white lines when you have to. Again, the more perpendicular you are, the better off you will be.

Rainbow coloured patches
If you can see the colours of the rainbow on the road, it is time to steer clear. The colours suggest that there is some amount of oily substance on the road and that can be enough to have you off. This seems obvious, but again, is easy to skim over. When riding along, watch for a neat row of rainbow patches, then you know something ahead was/is dripping oil, so you need to change lanes. When roads get wet, the rainbow colour is basically the only way to spot oil.

Algae
Again, this is an obvious one. But less-used surfaces, especially plaster-type and paver tiles near road edges are susceptible to that slimy green growth. It's fantastically slippery and forget the bike, you'll struggle to get your feet down. So when you're about to take that deep breath and park, watch for algae. Or else.

Pic from amazon.com