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The most common Houseboat journey are the O'Night ones(1 night).You could go to the Main Houseboat Jetty in Allepey and approach other travellers directly, you will need to get to the jetty around 8:30am - 9:00 am as this is the time when travellers will be inspecting Houseboats for their trip. You could also send this company an email and enquire if has any bookings by single travellers who may want to share a Houseboat, they also have Country Boats for hire,
http://naturevisionkerala.com/
Alternately ask around the Accom places in your immediate area where you are staying.
If you can't find someone to share a Houseboat and feel it's a bit too expensive to go on your own, consider
a Country Boat journey into the smaller canals, these are not very expensive and in my opinion a lot more interesting, they usually cost approx 300rps per hour, a four hour journey is ample.
Also IF you are staying in Kochi area you can do a Backwater Tour which I have heard is very good value for money, they pick you up and drop you back to your Accom and lunch is also included.
http://ktdc.com/tours.php
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You MAY want to also try posting a message here, you never know ?http://www.keralatourism.org/traveller-forum/
