Factors to take into account for comparison:
Policy wording (by no means standard!). Exclusions vary, are there any which affect your requirements?
Claims reputation.
Service standards, including ease of using web site.
Underwriting requirements (e.g. age of boat/survey)
Then you look at premium level including provision and cost of paying monthly.
Navigators & General, who have a significant portion of the market, is a trading name of Zurich. You will come across them in several guises with different premiums because it depends, amonst other things, on who is acting as agent or broker and the profitability of the "book" of business being written through that broker or agent, the underwriting criteria and policy wording they have negotiated and the "cut" they are taking.