I second what Ryan said - if you are going to do a trip at least 2 years in a row, buying a share could save you a lot of dollars. Example - buy a share for $5,000, pay $1500 annual dues this year and next, sell the boat share for $5,000. Essentially your week costs $1500 per year plus the pain of selling the share at the end. If you have a specific date of April 11-14 this year, it will be difficult to find a houseboat share with those specific dates available on such short notice. If you can be flexible, this is a good choice.
If renting from Aramark or APM, they may have early and late season discounts, so be sure to ask. I've also found it easier to make a deal with Aramark rental persons that are located on-site at Wahweap rather than the persons at the call center that you talk to if you call their 800 number. I can't believe how much their rates are for a 4-day rental now-a-days! Wow!