Cherai to Kumarakom distance is about 80km by road. There are two major routes options you can consider. Both goes via Ernakulam city. Both routes takes about 2 hours to reach Kumarakom from Cherai.
The first route is via Chertala and the second is Vaikom.
Route 1 (via Chertala): Cherai Beach ----> Njarakkal ----> Vallarpadam ----> Marine Drive ----> Pallimukku ---->Kundanoor-----> Aroor ----> Chertala ----> Thaneermukkam Band ----> Kumarakom
Route 2 (via Vaikom): Cherai Beach ----> Njarakkal ----> Vallarpadam ----> Marine Drive ----> Pallimukku ----> Kundanoor-----> Thrippunithura ----> Vaikom ----> Kumarakom
For both route 1 and 2 , you'll be heading south of Vypin Island along the Vypin-Pallipuram road. Towards the south of the island, the road will hit a large roundabout junction. Turn left towards Ernakulam City.
You are on the Goshree Road. After crossing two large bridges you'll enter the Bolgatty Island where you'll see a large roundabout junction. Keep straight , you'll cross another bridge and enter Ernakulam city center at Marine Drive. Turn right into Shanmugham Road.
This road will eventually join the MG Road. Turn right into MG road.
Keep continue on the MG Road towards Thevara, where you will turn left and join the NH47. At Kundanoor Junction you'll join the 4 lane highway (NH47).
From here on you have two major routes to chose from, ie via Vaikom or Chertala.
In either case you'll be going through the slow moving Ernakulam city traffic for about 10-15km.
For Chertala rote, turn right at the junction.
From here on the traffic moves fast till Chertala junction, thanks to the 4 lane route. The 4 lane ends near the Chertala bypass. Leave NH47 and take the left diversion to Chertala.
Further take the Chertala-Thaneermukkom road. After the Thaneermukkam Bund (a causeway across Vembanad Lake), the road joins the Kumarakom Road. Turn right to Kumarakom at the junction. That's about the route via Chertala to Kumarakom from Cherai.
For the route via Vaikom, at Kundanoor Junction, cross the NH47 and take the NH49. At Thripunithura take the Ettumanur (Vaikom) road. There are a few bypass short road to skip the Thripunithura town traffic. The road will take you to Vaikom. Cross the town and head for the Vechoor road. This road takes you to Kumarakom.
Apart from the above two routes, you can try a few other diversions. The roads are generally in good conditions for all the routes. The main concern is to avoid the city traffic, around Ernakulam side.
Boasted as the longest backwater cruise in Kerala, the Kollam to Alappuzha (Quilon to Alleppey) boat ride is an 8 hours affair. The route practically connects the two towns.