This study assesses the performance of metakaolin (MK)-based alkali-activated geopolymers as potential materials for underwater applications. Mortars were formulated using gradually higher sand volumes, which resulted in binder-to-sand ratios varying from 1:3 to 1:1.85 and 1:0.8. The alkali-activated solution (AAS) was produced using sodium silicate and sodium hydroxide. The AAS was adjusted to achieve a flow of 16 and 22 ± 1 cm while the setting times, plastic viscosity, washout loss, and 28-day compressive strengths were determined. Generally, an increase in AAS content prolonged setting time, while washout loss increased with higher sand content. Mortars with lower flowability exhibited higher resistance to washout, attributed to higher viscosity and stickiness of the matrix. The compressive strengths of mortars cast in dry or underwater conditions ranged between 23.4–54.1 MPa and 12.7–28.3 MPa, respectively; the residual strengths varied from 50.5% to 75.6%, mainly depending on the AAS-to-binder ratio.