Can High Water Mark be cleared?
From what I have seen reaching a High Water Mark means approaching some established limit. There are multiple "high water marks" in DB2 depending on which area you are talking about. Some relate to memory used for sorting/joining. Others relate to maximum agents allowed or differing heap sizes, or even package caching.

I'd recommend taking a look at this article on developerworks. Though it was written for an older version of DB2 (8.1), it still holds valuable nuggets for what parameters to tune when to get what you are looking for. I recommend taking a peek at it. If you search for high water mark on the page you find all the different instances I was referring to and which parameters tune them.

Also, take a look to see if you are running any hash joins and how many. This article is also helpful for understanding which parameters can be tuned that affect hash joins and certain usages of memory.