Battle against underfell sans in water ?

For those who play undertale, you know where the river boat guy drop you in Snowden place?, yes that spot of water before you meet the river boat guy , underfell sans surprise you with a surprise battle with him , the music of course plays as (Song That Might Play When You Fight underfell sans) , ufsans: hey kid! heheheh were gonna have great time -grin- -using red magic to float and rise up the water - -gaster blaster appear- get ready -battle engage-, / welp enjoy =3