We analytically determine the interface delocalization (or wetting) transition phase boundary in the limit of strongly type-I superconductors. In particular, within Ginzburg-Landau theory we derive an analytic expression for the reduced surface tension, Gamma(SC/N), of a type-I superconductor. We find that the truncated expansion Gamma(SC/N) approximate to 2 root(2/3) - 1.02817 root kappa - 0.13307 kappa root kappa (where kappa is the Ginzburg-Landau parameter) is so accurate in the entire type-I regime 0 less than or equal to kappa less than or equal to 1/root 2 that derivation of higher-order terms is unnecessary. We further derive an expression for the wall/superconductor interfacial tension which again proves accurate across a broad range of kappa values. These expansions allow us to locate the low-kappa interface delocalization phase boundary accurately, complementing previous numerical results for the wetting phase diagram.