Amazon offers Prime members the Disney Sofia The First 3" Sofia and Oona The Mermaid Dolls 2-Pack for $5.38 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, it adds $6.07.) That's $7 off list and the lowest total price we could find. Each doll includes a sea-themed accessory.
Orders placed via 2-day shipping by December 22 will arrive in time for Christmas. (This option is free for Prime members, but an extra $11.22 for everyone else.)