Tides are decomposed into ~37 principal constituents. M2 (period 12.42h) dominates most locations, driven by the Moon's gravitational gradient across Earth's diameter. Spring tides (M2+S2 in phase) occur at new/full moon; neap tides (M2-S2 out of phase) at quadrature. The Bay of Fundy resonates at ~13h, amplifying M2 to 16m.
The tidal force is the differential of the Moon's gravity across Earth: F_tidal ∝ 2GMm·r/d³. It creates an oblate bulge aligned with the Moon. Earth's rotation carries locations through this bulge twice daily. Friction in shallow seas dissipates ~3.7 TW — slowing Earth's rotation by ~2ms/century and moving the Moon outward ~3.8 cm/year.
When a tidal wave enters a narrowing estuary, it may form a bore — a turbulent wall of water. Resonance occurs when estuary length ≈ quarter-wavelength of the tidal period (L = T√(gH)/4). Amazon bore (pororoca) reaches 4m. Internal tides in the deep ocean — tidal energy converted to internal waves — drive much of ocean mixing.