OK, I did a little more digging. You can download the PredictWind offshore app and try it for free using a web connection. It is pretty good about giving you how much data you are requesting. For a small area (I tried the limits of Vancouver Island) you can get 4 wind models, wave data, observations, and predictions in <50kB. At their stated speed of 2.4kb/s that is about 3 minutes of air time if reception is good. They do specify that for boat use the outdoor antenna is required, so that makes the equipment buy-in about 2x the SiriusXM equipment. 3 minutes of airtime per download would give you 50 downloads in a month on the 150 minute plan, more than one a day. Have to add to that the PredictWind subscription itself.
Unfortunately, using the app, there are serious limitations compared to the PredictWind web app or iOS app. The high resolution wind predictions aren't available, and these are by far the most useful in the channels and close to shore. The Offshore app (required to use with Iridium) gives only the 50 km model, not the 25, 8, or 1km models. It is really only the 8 and 1 km models where you see the influence of terrain, very important in that area. I does show buoy and weather station observations, but then so did Sirius when I was there with it. Apparently you don't get any web access, and email is very limited. Tracking, SMS, and SOS I get on the Inreach which I already pay for (and it is far more portable). You don't get doppler radar or storm cell tracking, not essential but somewhat useful. You do get phone service, not available any other way.
Based on that experience, I'd say: I'd rather have PredictWind on their web app than SiriusXM. The satellite service in contrast isn't a clear win over SiriusXM for me, but costs about double. Of course others will have different ideas, needs, and priorities.
This isn't an academic exercise for me, I'm trying to decide what to install for the 5 months or so I expect to spend up there this summer.