Most ski areas will require CSIA membership, which is not the same thing as certification. Insurance is generally the stated reason.
I am a PSIA L3. While I am offered a substantial discount at, say, Schweitzer, I have never gotten any discount anywhere in Canada. Note that the discount, if any, offered by US ski areas may depend on your Cert level and your current employment status. It's up to the ski area.
Ski areas will not sponser you for a work visa. As noted in other posts, they have plenty of local/commonwealth "talent" to choose from. They don't need a US L1.
Despite my cert level, I entered the country as an engineer, not a ski instructor.
Crossing the border:
Be nice, answer the questions, don't lie.
US border agents are customs and immigration, and they are as picky as that dual designation might suggest. I prefer smaller border crossing stations. The lines are shorter. Sometimes, if you go through the same point several times per year, you might get to know a particular agent well enough to earn a friendly greeting, but they'll still check your passport on the system.
Many (but not all) Canadian border agents are customs only. They might subject your passport to a rigorous check, or they might just glance at it and ask if you have any guns or booze, or if you're leaving anything in Canada. Since I'm carrying British Columbia plates, it is possible they check on my vehicle before I even get to the booth. Then they already know who I am supposed to be and where I live. As a Canadian, I'm always asked what I purchased in the US.
If you're planning to work, you must have a work visa. If a Canadian customs agent figures out that you're entering to work, you'll be told to go away and attempt to enter at a point that also has immigration agents (if you're trying to enter at a minor entry point that has only customs agents). When I entered, I had phoned ahead to the actual point of entry I would be using to make sure I had all the necessary paperwork, including a job offer and a Human Resources Canada decision document. They were expecting me, and entry went smoothly. I was issued the necessary temporary work visa for a particular job with a particular employer right there at the border. If you're not prepared and they figure out that you want to try to get a job, they won't let you in.
Since I am a dual citizen now, I am always entering my own country, regardless of whether I'm headed north or south. That may help.