Planing amphibious vehicle (101) comprises engine (105) with internal cooling ducts (106). The engine is cooled first by air-water exchanger (102), and secondly by water-water exchanger (108). One or both exchangers may be located in compartment (119), separated from engine compartment (117) fully by bulkhead (80) (FIG. 4), or partially by bulkheads (118, 120). External water for exchanger (108) is drawn in from outside hull (109) through jet intake (114), the through inlet (115) in jet drive duct (116); and exhausted through outlet (123). Compartment (119) may also contain exhaust silencer (121). Numerals (103 and 104) denote grilles; (107) denotes cooling fan(s). FIG. 2 shows an alternative arrangement, with external water drawn from beneath the hull by pump (32). FIG. 1 shows front mounted radiator (2), covered by external flap (4) allowing air cooling to be shut down on water. FIG. 3 shows a water-water exchanger combined with the marine jet drive.