It's a solid base, but it has a few things that irck me.
The art is pretty good, but the enemies are kinda generic and a little washed out? The green slimes don't really pop much from the enviroment and most of the designs feel like they need a little bit more polish.
Mark is a really good idea, but I would really like a middle ground between "never shuts up" mark and silent mark, maybe make him not talk during the waves and only during exploration phase? or only make him throw a line when a wave starts and when it ends.
The gameplay is pretty solid, nothing to complain there.
Music and sound need work, because some of the sounds are way louder than each other.
Not bad, prettty good, but has some big issues that prevent it from reaching its high.