Hi Amy,

Welcome to the forum.
The cheapest accommodation i know of is at the mobile home park in Mora d'Ebre here is their website:

www.moracaravans.com  around 140 euros per week for a 3 bed roomed mobile home, includes water, electricity, gas, bedding etc, they are really friendly.

I'm not sure which yacht harbour you mean because Tortosa isn't on the coast, but there is one at L'Ampolla.

Hope that helps.

Jools