Perth Accomodation?

Travel Info
I'm from Singapore and would like to visit Perth in Oct. My intended stay is 5D4N and hopefully I can over these places:

-Rockingham
-Swan Valley
-Murray and Hay Streets

As this is my first visit, I'd need help on following questions:
1.) Reccomended Accomodation that is most convenient to above mentioned. Still, I'm open to suggestions. My budget to spend on accomodation is below $350(4 nights) for twin sharing.
2.) Any other Must-Visit places?
3.) I would like to have a 1 day private cruise with my partner, any idea how I may fulfil this in perth?

Hopefully, someone helpful will respond. Its been a wish of mine to visit Australia with my special one.