Underrated Cities in Spain Near the Water?

I'm not sure how off the the beaten it is (maybe a little), but I would suggest Tarragona. It's easily reached from Barcelona, Roman ruins, close to Priorat wine country, close to Sitges, fascinating old city.

Tossa de Mar !!!
Only an hour and half by bus from
Barcelona . Not really a city , a town , right in water with gorgeous ancient still entangled ramparts on hill overlooking town .
We spent 4 days there and loved it !

Google for amazing photos

What sort of places interest you? Do you care about museums, historical sights, architectural style, cultural events, etc.? Do you have a size preference? Are there geographical limits? If you are looking for one or two places where you might spend one or two days (each), you'll probably want those places to be reasonably near places already on your itinerary.

Well, Spain is a very heterogeneous country and cities, customs, landscapes, climates vary a lot in just a few hundred kms. On the north, I´d recommend San Sebastian, Gijón, Oviedo, La Coruña, Pontevedra (cities, not towns), all close to the water...but take into account that the north is rainy, wet, green and mountainous, and is much less touristy than the Mediterranean. On the Mediterranean, Cádiz, Huelva, Almería, Tarragona...

for cities, we like Valencia, San Sebastian, and Vigo (I know - not a popular choice), Santander
for charming seaside towns - Getaria, Sitges, Tossa de Mar, Cadaques.

On a future trip, we plan to explore Galicia more. The beaches are really beautiful.

Oh, and don't rule out Mallorca.

Some more information in what interests you, how long you have and where you will be when "on" the beaten path might help.

If you are in northern Andalucia, then Ubeda is worth visiting (it's not at the seaside, mind).

Further east and on the coast is Cartagena, which is worth a day or two if you are in that area - but I wouldn't cut short time in Sevilla or travel all the way from the Basque Country just to spend a day in Cartagena. It is more a third or fourth trip sort of place.

I think I saw on this site that RS doesn't mention (or only in passing?), Valencia. If true, then that might qualify (although note it is mentioned in all general Spain guidebooks I've seen so I wouldn't really call it "off the beaten path". Perhaps only for Americans?).

Similarly, Cadiz is, I think, underrated by those who tend to skip it in favour of the nearby "White Towns". Personally, I think Cadiz is one of Spain's top cities to visit, but it doesn't get nearly as much mention as some others on here.

If you are looking for a smaller seaside place, then Mojácar, Almeria is nice with the interesting old town above in the mountain and the pleasant beach part of the town below.

Some cities we have visited in Spain that are on or near water and worth a trip are Cadiz, Baiona, Comillas, Ribadesella, Begur, Acquablava and the towns in the Rias Baixes just north of Portugal, especially O Grove.

To you, does off the beaten path mean accessible by train or bus, or places where you need to have a car to get to. How small do you want to get? There are literally hundreds of really small towns worth visiting, but you need a car to get to, and to get to nearby sights. The smaller the town, the more likely you’ll need to drive to get to the worthwhile sights. As someone already asked, what are your interests. I second Nick‘s suggestion of Mojácar. Getaria and Zumaia were nice as was the area around Ribadeo.

I had never heard of Alicante when our cruise ship went there.

i found it interesting.
i guess the British go there a lot.

There are so many beautiful coastal towns near Barcelona. Sitges is probably my favorite one, but you should also visit Tossa de Mar, Begur, Calella de Palafrugell or Cadaqués.