Hello Rosa11969 ,
Having read your tour description, I would hardly call it going from Siem Reap to Phnom Penh by boat.
The only way from Siem Reap to Phnom by boat is to travel down the length of the Tonle Sap.
You are, in fact, not going down the Tonle Sap to Phnom Penh but taking a fairly long cross-country road trip from Siem Reap down National Road # 6 - the main road to Phnom Penh, then on to National Road # 7 and stopping briefly at Skuon to see the spiders and then on to, most probably, Kg Cham, depending on water levels, to board a vessel on the Mekong River.
The section of the Mekong River from Kg Cham or a bit south of there, depending on water levels, is interesting. Particulary so as you turn in towards Phnom Penh and the confluence of the Mekong, Bassac and Tonle Sap rivers.
Best regards. Ian/Angkorlad