Your presentation was flawlessly smooth — I'm impressed.
Literal
Standing-board [to-に] water [possessive-の] like presentation, impressed.
立て板に水 is an idiom meaning 'like water on a standing board' — describing fluent, uninterrupted eloquence. 如き (ごとき, like) is a classical/literary form of のような. おそれ入りました is a humble expression of being impressed or grateful.