Ross Lake is a reservoir that generates much of Seattle's electricity, so it is regulated by power need. You can see a description and current lake levels here
https://waterdata.usgs.gov/wa/nwis/uv?station=12175000. I assume you want to fish the BC end of the lake? If you drive down to the WA end, the resort rents boats (there is no boat launch on the US side, so you must rent a boat here if you come in from this end). Here is the rental info:
http://www.rosslakeresort.com/equipmentrental.htmlI just noticed this little gem on the Skagit Valley Provincial Park website: "Reservoir Information
Ross Lake campground is located on a reservoir. The level of the water in the reservoir fluctuates greatly. July and August are the only months when the lake is at full pool. Other times there is no water at all on the Canadian side."
Is it July that you are planning on going? Hopefully the lake will be a full pool by then - or you can fish the US side.
As for boat rentals, I surprised myself but found this: "on a trailer for pick up in Port Coquitlam"
https://www.bcboatrentals.ca/. I'm not sure, but their boats may be a bit too big for this purpose.
I should probably be working, but instead I am trolling fishing forums while counting down to the June 1 opener for spring Chinook on a local river.