The Summer months are a fantastic time to explore the Ribadeo (or Eo) estuary. There are a number of ways to do this, but probably the easiest is by the tourist boat that operates from the port in Ribadeo. The Nuevo Agamar costs 5€ per person and leaves Ribadeo every half hour. It travels across the estuary to Castropol and then on to Figueras, before returning to Ribadeo via the puente de los Santos (bridge across the estuary).
In Figueras there is a shipbuilding yard called Astilleros Gondán. It is well known in Spain for building support vessels for the oil industry. From the Nuevo Agamar, you travel right under the boat that is under construction. There is also a nice restaurant in Figueras called Peñalba so you can get the boat from Ribadeo or Castropol, travel to Figueras, have lunch and then get the boat back to where you started from, after lunch. Alternatively you can get the boat to Castropol and have lunch in el Rison
For those of you that prefer more of a challenge, you can hire a self drive boat and explore the estuary yourselves. These can be hired from La Flote in the port of Ribadeo. They are small boats with outboard motors and prices start at 30€ per hour for the smallest engined ones. A whole day costs 130€. They are great if you want to get to the huge sandbank that appears at low tide. This is great for a picnic, playing football or just relaxing. You can anchor the boat and do what you want.
You can also explore the estuary on foot and it is a birdwatching mecca all year round. Probably the best walk is between Figueras and Castropol. There is a footpath that winds its way around the edge of the estuary and you get great views out over the water towards Ribadeo and Castropol. In the Winter it is a great place for wildfowl and during the migration there are lots of waders, Spoonbills and Egrets. There is normally a wintering Osprey. Have a look at Castropol, Asturias