Complex sentence: The meaning of Rabbi Akiva's exhortation is that the Sages should not declare that there are two types of water, since there are not... lest you endanger yourself because of the sin of separation.
Split into simpler sentences: The meaning of Rabbi Akiva's exhortation is that the Sages should not declare that there are two types of water. Since there are not... lest you endanger yourself because of the sin of separation.