Given the vast distance of ALR such a sky train line would have to traverse and the relatively small population it would serve, along with the fact that a direct connection to the ferries would still require a rather long spur, that is a terrible idea.
It would take at least 35km of guideway to reach White Rock from Brighouse. A further 10km or more would have to be built to reach the ferries if you wish to add that as part of the deal, which would be a major waste of money given all the other corridors skytrain money could be spent on.
A good idea is to replace this 50+ year old seismically unsatisfactory tunnel with a new road structure that would incorporate a rapid Bus system to White Rock / South Surrey.
Even if traffic volumes are decreasing slightly, there is still an asinine bottle neck coming northbound when it funnels into a single lane. I have to do it occasionally and each time it literally takes over an hour to travel about 3km distance as 5 lanes funnel into a single one. No excuse for that.
At the very least a replacement should be 6 lanes wide with bus / HOV lanes. My preferred replacement would be akin to the new Port Mann, with a 2X3X3X2 C/D system and rapid bus system.