Well I have to say the 1st time I was asked to install a fixed dish on a houseboat I was REAL skeptical. Doing it for an hsp, rsp or inhouse would be a no go for me. Just because of their BS.
However as a retailer I've done more than I can count, fixed & auto dishes, mobile houseboats & permanent houses on the water. It is very surprising how well fixed dishes hold up on water. It really depends on the size of the houseboat or permanent house on the water & how it is anchored. Some will turn significantly with changing water levels, others especially fixed permanent houses not so much.
Within the last 2 or 3 weeks I installed a 3 tv D* on a mobile houseboat 80 X 18 with 2 levels parked at the dock. You could feel the movement of the boat. It passed install verification without a hitch & never lost signal the entire time I was there. This customer knows the drill & occasionally takes it out on the lake. He previously had dish which I installed about 3 yrs ago & never heard a peep from him till I switched him to D*. This is something they only use on the weekends. It is no big deal to them to re-tune when necessary, especially when it's only needed for the weekend. If signal is gone they take a few minutes with 1 at the dish & 1 at the tv using cell phones & their good until they leave.
Most customers understand the need to be able to re-tune their dish on their own, otherwise I wouldn't fool with them. Also I have yet to see an auto dish that will pickup 99 or 103 on D*, besides that most will only see 1 sat at a time. Regular dishes are by far the best way to get all programming.
I do have 1 customer that calls 3-4 times a year with a mobile houseboat. He has a land based system with flooded cable laying in the water coming to the boat, also there is nlos for the auto tracker when boat is at the dock. Most of his service calls are easy fixes, wrong surround sound input, receivers losing authorization due to being without pwr for weeks on end. However he always gives me $100+ when ever I go out there. I always say that is too much, his response is you'll just owe me 1 for the next time, however he tells me that every time & still pays me very well.
From the best I can tell the op has fixed house on the water similar to this.
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If this is the case your best bet is to find a retailer who will do this for you. Most of these types of homes are very heavy & stable with supporting systems that will not turn during water level changes. For me personally I would not think twice about installing a fixed dish like the 1 pictured above as long as there was an understanding there would be a fee for me to re-tune or the customer willing to do if needed. On a home like this I would bet it would be rare to have to realign. Years ago I would have said no way, but working with my lake customers has given me a whole different outlook that yes it is doable. I do however draw the line with sat internet, it has to be land based regardless.