The diffraction of tidal waves (Poincaré waves) by islands and barriers on water of constant finite depth is governed by the two‐dimensional Helmholtz equation. One effect of the Earth's rotation is to complicate the boundary condition on rigid boundaries: a linear combination of the normal and tangential derivatives is prescribed. (This would be an oblique derivative if the coefficients were real.) Corresponding boundary‐value problems are treated here using layer potentials, generalizing the usual approach for the standard exterior boundary‐value problems of acoustics. Singular integral equations are obtained for islands (scatterers with non‐empty interiors) whereas hypersingular integral equations are obtained for thin barriers. Copyright © 2001 John Wiley & Sons, Ltd.