Ocean Under the Ice — Robert L. Forward

INVASION

When the first interstellar expedition reached Barnard's Star they found Rocheworld, and discovered therein the flouwen, giant amoeboids with a flair for math. Almost immediately, the medium-brained but high-tech humans from Earth forged happy alliance with the big-brained, lovable blobs, and a man-amoeba team set out for Zulu, the ice-shrouded moon of a gas giant in the Barnard system!

There they discovered deep oceans under the ice, and gigantic hot water geysers that spouted plumes hundreds of miles into space. The plan is for humans to investigate the huge geysers using amphibious tanks, while the flouwen, who live only for math and surfing, ride a geyser to its top.

Neither man nor blob had expected that Zulu would have a native sophont species who thrive on the nutrients around the vents that spawn the geysers -- and that these natives would interpret an alien approach as an assault on their food supplies....