If you want to go from Phnom Penh to Siem Reap, your two direct ground options are the bus or the river/lake ferry. The bus is about 8 hours on "an intermittently rough 314 km" Route 6. The ferry is about 5 hours on a relatively calm lake. I paid my $25 and took the ferry.
The Rambo 7 is a 68 seat diesel river ferry which noisily belches dark smoke. Half the passengers including most of the tourists opt to ride on the steel roof, which is nice because the two by two seats are at approximately 24" pitch. Hope you don't mind bumping your knees. I opted to ride inside as I figured there would be less chance of my losing my belongings that way. The roof is much more picturesque, and after I staked out my seat below I went topside for a while.
The first couple of hours out of Phnom Penh provide the best views. There's ample opportunity to snap photographs of stilt buildings, fishing boats, canoes, and houseboats, which is why most of the tourists were braving the tropical sun. The ride is smooth enough to make clambering onto the roof and back down below a reasonable risk.
After the boat passes Kompong Chhnang the river opens out into a lake and the view becomes much less interesting. At times the riverbanks are so far away as to be invisible, and all there is to be seen is brown, choppy water. This gets boring pretty fast and at that point one's best option is to go back to one's cramped seat and get some sleep.
About half an hour from the Siem Reap end (during the dry season, anyway) a small tugboat arrives to tow the Rambo 7 through a winding, narrow channel, full of houseboats and other small craft. It becomes clear why the boat has such a shallow draft as there are people wading chest-deep in the water pushing their canoes around. Grounding is not unheard of.
These ferries would obviously be shut down immediately in an industrialized country. There are no lifevests to be seen, and there's no safety briefing before the ferry gets under way, in Khmer or any other language. Our boat had a permanent list to port. On the other hand, by local standards, they're reasonably safe. There are occasional annoying mechanical failures, and sometimes irate fisherman take shots at the ferry when their nets get snagged, but none's drowned lately. And, as frotz said a while back, "So far as I can tell, the attitude in most of the world is 'They're boats. Shit happens. Deal.'"
Apparently similar vessels are used on the coastal run between Thailand and the southern Cambodian ports. This strikes me as a questionable idea in any but the calmest seas.
One of my fellow passengers was wearing a Cabot House T-shirt which said "fish-slapping the river since 1985". I didn't ask.