Sunken is completely standalone.
It's based closely on the Trophy ruleset and is set in the vaguely same world as Trophy. So, if you like the tone and gameplay of Sunken, you'll enjoy Trophy – and vice versa. Most elements should crossover between the systems fairly well: incursions, character types, etc. Maybe most importantly, you can utilize many of the resources and actual plays that exist for Trophy as you prepare to run Sunken.