Water running can be much harder than the track. If you do it right, regular land running is gonna feel like a day off. And yes, the cooling effect of the water can buy you some time when the workouts get tougher
-- you can get more blood to active muscle,
-- and higher lactate levels for any given RPE.
That might explain the nausea.
You also have the assist on blood getting pumped back to the heart. But water running is really about shifting the demand to the peripheral.
Intervals in the pool are a bit overrated though. I think some coaches push them because they haven't actually run in the water themselves.
I think you're finally ready for steady running, say somewhat hard to hard, for 60'. My hardest ever was hard effort for 90'. A few times it was hard to get out of the pool.
If you wanna make the interval workout a bit tougher, try shorter reps, shorter rests. 20" w/ 5" rest, that sort of thing. You can do a lot of work/rest combinations in the pool that you can't do on land.