Just for me personally about crawlspaces:
If it is relatively dry, plastic sheeting or pea-gravel, has been sealed up, and I can fit, I usually don't have a problem going in them, and I'm a big guy. 6'7" and 290.
If they have been open, as in a vent has been knocked out or the metal door is open, I'm really leery about animals (snakes in particular love crawlspaces) but I'll usually still go in them. Water usually collects around the outside walls if it collects at all.
If it has standing water in it, I absolutely WILL NOT go in it. For one simple reason. Most electrical on trailers/modulars is done underneath as well, something could be hanging down in the water running a current through it and you wouldn't know it til you touched it.
If it just has the plastic skirting around it, no need to go under it really just throw some fish rods up under there and attach your cables, pull them out the other side or whatever. I usually drill down through the floor, run my cables DOWN, then hook them with my rods and pull them to me without having to go under it.
But like I said most cinder block foundation crawlspaces, you either have to go in or run everything on the outside of the house.