Embalse de Ullibarri-Gamboa (fuera del parque provincial)

What the official source says

«En el resto del embalse que no sea parque provincial, podéis bañaros con vuestro perro en cualquier temporada del año.»

• TypeDog-friendly reservoir
• Seasonat any time of year (outside the provincial park)
• TownÁlava

Within the provincial park, animals are prohibited from entering the water and the bathing areas (Garaio Norte, Garaio Sur and Landa) and a lead is compulsory in the rest of the site; green route with no dogs off the lead.

Source: Official tourism · 2019. Rules change every season: check them before you go.