This is a tricky one, so bear with me: the answer is, we’re not entirely sure.

We know that the Sealords of Braavos are elected (for life) and we know a little about who elects them:

Neither prince nor king commands in Braavos, where the rule belongs to the Sealord, chosen by the city’s magisters and keyholders from amongst the citizenry by a process as convoluted as it is arcane. From his vast waterside palace, the Sealord commands a fleet of warships second to none and a mercantile fleet whose purple hulls and purple sails have become a common sight throughout the known world.

Part of what makes this complicated is that the electorate is composed both of magisters and keyholders, and it’s not clear whether all keyholders are magisters but not all magisters are keyholders or whether the two classes are completely separate. We do know that both magisters and keyholders form quasi-nobilities, with the Antaryons and Prestayns as two well-known houses of the former (the current sealord is an Antaryon, for example), and the Reyaans as one well-known family of the latter. Another complication is that the keyholders are officials of the Iron Bank of Braavos, which suggests something of the interconnection of state and corporation as was the case with the Dutch Republic and the Dutch East India Company. 

There’s also a suggestion, and this is more speculative, that there’s also a legislative body in addition to the Sealord’s executive. The books refer to a Hall of Truth (and sometimes a Palace of Truth, although it’s possible they could be two separate buildings) which is distinct from the Sealord’s Palace, where keyholders are “summoned to the Hall of Truth to vote.” While this could just be where the elections for the Sealord are held, the phrasing suggests otherwise (given that the choosing of the Sealord is a much more drawn out process than a sudden summons would suggest). Also, given how much Braavos loves its liberty, it would be surprising if they were less democratic than Lys, Myr, and Tyrosh.