By water, the distance from Kurun to Jakaar is about 190km. The river is about 120km of that distance. All of the following rules are based on the time it takes to travel this distance.
You may sail for 14 hours/day. This is assuming that you will be stopping for the night to rest, giving the GM a chance to add random encounters on the shore. To flatten things out, and follow a similar table as riding/marching, go:
Day's Sailing. 45km/day Working the Oarsmen. 60km/day (treat as forced march for the rowers) Death Beat. 70km/day (treat as death march for the rowers) When traveling downstream, treat each result as one level faster. When traveling upsteam, deduct 10km/day from distance traveled.
At this rate, it would take just under 3 days to reach Jakaar from Kurun utilizing a death beat on the third day. From Kurun to Jakaar, it will take a max of 6 days. With a death beat every day, you can reach Kurun in just just 4 days.
From here, implementing weight restrictions comes into play. This would be entirely up to the GM. An easy rule would be: for a vessel that is 75% loaded or greater, deduct 10km/day from the travel time.
This is assuming all oars manned. If your players try to skimp on rowers, you can always sub their PCs in to the position to make things work, or modify the bonuses (half rowers would be 45+5 or 45+10 for death drums, etc).
If you like the extremely quick travel potential that is allowed by the waters- you might want to modify things to:
Day's Sailing. 60km/day Working the Oarsmen. 80km/day Death Beat. 95km/day When traveling downstream, treat each result as one level faster. When traveling upsteam, deduct 20km/day from distance traveled.
Utilizing the oarsmen, this would allow a two day trip from Kurun to Jakaar, utilizing a death beat the second day. The reverse would take a max of 5 days, just sailing. With oarsmen, it would take 3, and about 2.5 with a death beat.
This is also assuming the captain either works in shifts with a second navigator, or takes small breaks through out the day to rest, so that he may remain awake at night and get the ship to its destination as quickly as possible.