If I set haskell-process-use-presentation-mode to t this make no difference if it is set to nil. E.g. no matter what value is used, when I run haskell-mode-show-type-at it shows me colourful type information in echo area, and also prints to *** Messages *** buffer.
I'm not sure what Presentation mode actually is, but I expected that Emacs will create some kind of popup instead of echoing type information when mentioned variable is set to t. Currently I have haskell-doc-mode enabled, which almost instantly replaces nice colourful type information from interactive haskell mode with plain colourless text; I hoped that presentation mode will help with this.