Next to useless!
Simplest option is a Lidl/Aldi 1Kg powder extinguisher (with pressure gauge) - you'll need to wait until they stock them but they are the cheapest option (usually under £10 each). Powder is a good extinguisher but makes a serious mess (I wouldn't have one indoors!). You need two in any water craft (when you need one you REALLY need it to work!). Ideally mounted horizontally to avoid powder compaction (or give them a shake once a year when you check the charge).