An independent Alberta is guaranteed sea access under UNCLOS Articles 124 to 125.
Stated by: Commonly stated by Alberta independence advocates
Summary
The UN Convention on the Law of the Sea does give landlocked states a right of transit to the sea, and it even mentions pipelines. But the same articles make the terms of that transit subject to agreement with the transit state, and they affirm the transit state's full sovereignty over its territory. So the treaty would not compel British Columbia or Canada to build or permit pipelines. Presenting it as an automatic guarantee overstates what the treaty actually requires.
Evidence
UNCLOS Part X gives landlocked states freedom of transit, but Article 125 says the terms must be agreed with transit states, and Article 125(3) preserves the transit state's full sovereignty over its territory.
UN Convention on the Law of the Sea (Part X) (opens in a new tab)
Legal experts describe the claim of a guaranteed pipeline route through B.C. as misleading, because access still has to be negotiated with the neighbouring country.
The Week (constitutional crisis analysis) (opens in a new tab)