Thames River Services, Westminster Pier, Victoria Embankment, SW1 (tel. 020/7930-4097; www.westminsterpier.co.uk; Tube: Westminster), concerns itself with downriver traffic from Westminster Pier to such destinations as Greenwich, St. Katharine’s Dock, and the Thames Barrier. The most popular excursion departs for Greenwich (a 50-min. ride) at half-hour intervals between 10am and 4pm daily in April, May, September, and October, and between 10am and 5pm from June to August; from November to March, boats depart from Westminster Pier at 40-minute intervals daily from 10:20am to 3:30pm. One-way fares are £9.50 for adults and £4.75 for children 15 and under. Round-trip fares are £12.50 for adults, £6.25 for children. A family ticket for two adults and as many as three children costs £32 round-trip. There’s no service on December 24, 25, and 26, or January 1. City Cruises (tel. 020/77-400400; www.citycruises.com) operates a near-identical service at similar prices from the same pier.