This verse of Mark is similar to Matthew, chapter 10:42, but not in Luke. Matthew had the gift of water to the little ones, not the disciples. Mark indicated that Jesus said that whoever gave them a cup of cold water to drink (Ὃς γὰρ ἂν ποτίσῃ ὑμᾶς ποτήριον ὕδατος), because they bear the name of Christ (ἐν ὀνόματι, ὅτι Χριστοῦ ἐστε), would be rewarded. Jesus spoke with a solemn pronouncement (ἀμὴν λέγω ὑμῖν). They would not lose their reward (οὐ μὴ ἀπολέσῃ τὸν μισθὸν αὐτοῦ). Why would they lose their reward anyhow? Once again, Jesus spoke with authority. There is an explicit mention of Christ (Χριστοῦ) that was rare in Mark. People would not lose anything by giving cold water to his Christian disciples, a very small gesture.
Second Isaiah has Yahweh call his servant Israel, not Isaiah. Yahweh was going to be glorified in his servant. However, the servant responded that he had labored in vain, spending all his strength on nothing but useless things. Nevertheless, this servant was willing to say that his cause was with Yahweh. He expected his reward from his God.