Quick Answer
If the Moon's mass doubled while its orbit initially stayed similar, its gravitational influence on Earth would roughly double. Tidal forcing would become stronger, although actual coastal tide heights would still depend heavily on ocean basins and resonance. The Earth-Moon system would also evolve differently because stronger tidal interactions transfer angular momentum between Earth's rotation and the Moon's orbit.
The Moon is unusually large relative to Earth, which is one reason its gravitational effects are important. Doubling its mass would strengthen those effects, but not in a simple global two-times-everything way.
Tides would become stronger
The tidal acceleration produced by a body depends strongly on its mass and distance. If lunar mass doubled at the same distance, the lunar component of tidal forcing would approximately double. Actual high and low tide heights would not necessarily double because coastlines, ocean depth and resonance determine how forcing translates into local water levels.
Earth would lose rotational energy faster
Ocean tides dissipate energy through friction and other processes. A stronger Moon would generally increase the strength of tidal interactions and could accelerate the transfer of angular momentum from Earth's rotation into the lunar orbit, although the exact rate would depend on Earth's oceans and the changing Earth-Moon distance.
The Moon's orbit would evolve
The Earth and Moon orbit their common center of mass. Increasing lunar mass would shift that barycenter and change the dynamics of the pair. If the Moon were magically made heavier without changing its position or velocity, the system would immediately cease to be in the same dynamical state and would begin evolving toward a new orbit.
Coastal ecosystems
Intertidal habitats are built around the timing and strength of tides. Stronger tidal cycles could expand or contract certain habitats and change the transport of nutrients and sediment. Species that use lunar cycles as reproductive or behavioral cues could also experience stronger environmental signals.
SpeculativeA precise prediction for global sea-level changes is not possible from lunar mass alone. Local tide height depends on the geometry and resonance of each ocean basin, so some coastlines could experience much larger changes than others.
Approximately 2×
Lunar tidal forcing
Strongly geography-dependent
Local tide height
Different long-term braking
Earth rotation
The bottom line
A twice-as-massive Moon would make the Earth-Moon system more dynamically influential, especially through tides and long-term rotational evolution. But 'twice the Moon' would not mean every coastal tide simply becomes twice as high.
A note on our approach: Every article on WhatIfLab separates what current science establishes from what remains genuinely speculative. Where we cite a figure or finding, it reflects published, mainstream research at the time of writing — not a prediction dressed up as fact.