Venice to Salerno by train along Adriatic- take the scenic route

I can piece it together by asking for Mestre to Termoli

There isn't a better way to pick the slowest and longest route. Without mentioning the 2 transfers to get from Termoli to Salerno.

It's hard to know from just the schedules.

If you open the intermediate stops menu on the search results page of trenitalia.com, you can see where each train is stopping along the Adriatic coast.

Otherwise go on viaggiatreno.it click the UK flag and zoom in the railway map of the Booth. Termoli is in Molise.

I think that's the best place to get a train

Why would you think so? The best place to get a train is the station closest to where you are staying, i.e. Venezia S. Lucia if you are staying on the islands.

will actually see the coastline

Thanks to the Apennines Mountains, south of Rimini You will actually see a lot of tunnels and the coastline when the train will be getting closer to the stations along the way. Now you know why that line was slower even before high speed trains.

Do yourself a favor and take the direct Frecciarossa just past noon, and do Venezia S.L. to Salerno without changes in 5h 45min. As a second best, try a combination of Frecciarossas with a change, probably in Rome, and do it in 6h 45min. Scenery? The eerie hills around Orvieto are better than sea, IMHO.

You really want to do the Adriatic with a stopover in Termoli? First, Venice to Bologna and then try to find a direct train to Termoli. No coastline before Rimini - there is no line along the coast. (Somebody could suggest Venezia-Ferrara- Ravenna-Rimini, shorter but slower and only a short and distant glimpse of the sea. Rimini to Termoli is mostly along the coast; the original line had some stretches literally on the beach, being slowly rebuilt inland to reduce exposition to sea in bad weather. Still a slow line. The faster combination Venice-Termoli is 6 hours.

The real dog is Termoli to Salerno. Very few options during the day, the shortest being Termoli-Foggia-Caserta-Salerno with two changes; 5 hours but very limited choices, so you will be spending all the day on this leg. I see longer options involving a lot of regional trains (for example Termoli-Foggia- Potenza- Salerno) or bus through arcane places. So looking at the sea will take two whole days.

There are 2 direct trains a day between Venice S. Lucia and Termoli, at 6:53 and 14:52. Both Freccias do the trip in 5 hours and 42 minutes.

It's the Termoli-Salerno leg that takes ages, I'd rather get a car and enjoy the 3 hours drive through the mountains,