Booze cruise is a British colloquial term for a brief trip from Britain to France or Belgium with the intent of taking advantage of lower prices, and buying personal supplies of (especially) alcohol or tobacco in bulk quantities. This is a legally acceptable process and should not be confused with smuggling.
The term is also used in other countries to refer to a pleasure outing on a ship or boat involving a significant amount of drinking, or an outing to purchase large amounts of alcohol in bulk for a party or outing. It probably originated during Prohibition, when Americans would take "cruises to nowhere" to enjoy alcohol, which could legally be served on board once outside American territorial waters.
Generally, alcohol and tobacco taxes are lower in France than in Britain. Economically, it makes sense for people to buy their supplies of wine, beer, spirits, and tobacco in bulk in France instead of Britain. There is keen competition between ferry operators and the Channel Tunnel Eurotunnel Shuttle. The day fares to Calais are normally around £60 per vehicle, but are sometimes discounted to as little as £10 at off-peak times. Since beer often costs little more than half the English shop price, at worst, the savings defray the cost of a day out.
Serving this market has become big business around the major ferry ports of Calais, Boulogne, and Dunkirk in France, and Ostend in Belgium. Even longer routes from Cherbourg, Normandy and Brittany generate business, boosted by ferries from Ireland where alcohol duties are even higher. The exit route from the Calais ferry port passes several large warehouse retailers (English-owned) that serve the market, and some large British supermarket chains have alcohol-only branches selling bottles identical to those in Britain, but at deep discounts. The French have opened an enormous shopping precinct adjacent to the Channel Tunnel (Cité Europe) that attracts large numbers of British shoppers.