I did Bilbao to Santander around three weeks ago. It is scenic but I was surprised how little of the coast that you saw.
Just several tangential points:
It is cheap-about 10 Euros from memory
No reservations. Busy at both ends of the line but quiet in the middle. I think I was the only one who travelled all the way.
You can buy your ticket online (Renfe website) but the barcode won't open the gateline barrier. Not a huge amount of point tbh as the Bilbao station has a staffed ticket office.
The specific platform gate at Bilbao for the Santander train is opened (for free entry) around 30 minutes before departure.
Surprisingly little English understood
We did Bilbao La Concordia to Santander and then Santander to Ribadesella on Friday 30th May. We were using Interrail passes.
QR Code on the pass wouldn't open the gateline barrier at La Concordia and the first, young, chap we asked said we needed reservations for the train with an Interrail pass, then an older chap, who turned out to be the guard/ticket inspector/jack of all trades on the journey over-rode him saying that reservations weren't needed. This was my understanding as it is basically a commuter service with no means of reserving seats.
Along the way this chap came and told us in English that we would have to change trains at somewhere I didn't catch the name of - he had translated "I will tell you when we get there and help you" on his phone which he showed us. Can't remember exactly where this was but good to his word he shepherded us and all the other passengers off of the train and watched as we all walked across the line to board the train on the other platform which was going to continue the journey while he and the original train were the return service I assume back to Bilbao.
On arriving at Santander our passes wouldn't open the gateline. After finding someone to let us through I went and had a word with the lady in the ticket office who said to come and find her when we needed to get through the gateline again. We went off and found some lunch and enjoyed the sunshine and returned to Santander station for the 15:28 which was going to take us to Ribadesella. Aforesaid lady nowhere to be found and no staff around just a security guard who didn't want to know when I asked him how we got through the barrier. Then we found an Australian couple also travelling on a pass and wanting to take the same train who had also been having problems with their pass and gatelines. We then found a member of staff who let all four of us through rather grudgingly. The Australians said it wasn't the first time they'd found problems getting through the barriers on trains that didn't need reservations - the codes on the reservations worked fine.
I'd agree with Djgr that the ride is scenic, and fun to do, but I too was surprised we didn't see more of the coast. It felt a bit like taking the two coach service from Bletchley to Bedford but with better scenery.
We loved the "Thomas the tank engine" sitting at La Concordia - tried in my not very good Spanish to get a laugh from the younger chap by saying that in England all little engines like that were called Thomas but my joke fell flat.