IN THE UK
In London there are three annual top-level watercolour shows that anyone can enter. I enjoy attending these to see excellence and new ideas in the medium.
The RI, Royal Institute of Painters in Watercolour, holds an exhibition early in the year where members exhibit up to four large paintings and two small paintings, and remaining space is allocated to non-members' work juried by the RI. This exhibition is the only solely peer-reviewed top-level competition for watercolourists in the UK, and only non-members' paintings are judged. The RI is based at Mall Galleries, just off Trafalgar Square.
The RWS, Royal Watercolour Society, hold two annual exhibitions of members' paintings and also holds a solely non-members' show early in the year juried by a panel, in 2016 consisting of: a gallery director, an arts writer, the President of the RWS, a member and an Associate member. The RWS is based at Bankside Gallery near Tate Modern.
The third event, The Sunday Times Watercolour Competition, is a privately-sponsored open show; in 2015 it had been running for 28 years. It is juried by a panel, in 2015 consisting of: the Sunday Times newspaper critic and writer, the Surveyor of the Queen's Pictures, a magazine editor/art critic, and two artists who had been selected the previous year. It is held at Mall Galleries, usually in September.
The main resource for keeping informed about the UK watercolour scene is The Artist magazine, which covers all media.
Almost all regional societies in the UK are mixed media societies. The Pure Watercolour Society is based in the Cotswolds and supports traditional British watercolour.
IN THE US
The US has two national watercolour societies, the AWS, American Watercolor Society and the NWS, National Watercolor society. Some artists are members of both societies; I mostly know about the first. The AWS is based in New York and holds an annual open show, with the catalogue of all paintings exhibited available digitally - always worth a look. As I understand it, all paintings are juried for the show, both members' and non-members', with members voting during the year for whom they wish to appoint as judges for that year; names of those on the jury of selection and the jury of awards are published in the entry prospectus.
The US has a dedicated watercolour publication, Watercolour Artist magazine, which often features the award-winning paintings from both national and the US's many regional watercolour societies.
The Art of Watercolour magazine is published in France (also available in French) and features Europe but also the world, so it is a good way to see what is happening globally. An exciting development is the number of new national watercolour societies being established throughout Europe and the world.
A lot is happening in the East, with China bringing its heritage of attention to brushstrokes and respect for the fluidity of the medium to the global scene with stunning masterful works. There are two international biennial competitions in China, one in Shanghai and one in Shenzen, both showcases of excellence attracting international judges.
It is also worth mentioning here that when you attend a workshop, especially an international one, you learn from the fellow students as well as the teacher, including advice from other instructors they have studied with and snippets of what's happening in other parts of the world.