In line with what Tertullian would write elsewhere, he expected that there would be enough water to fully immerse the person being baptized. This would meet the criteria mentioned in The Didache of being immersed in moving or still waters. Not only that, but Tertullian links the “sea or a pool, a stream or a fount, a lake or a trough” to the Biblical prescription of baptisms in bodies of water. Also note the emphasis Tertullian placed on the Holy Spirit sanctifying the waters after the invocation of God (by the one baptizing) and the sanctifying properties he said were granted to the water. This could be a germ of the later development of the regenerative or justifying nature of the water.
And accordingly it makes no difference whether a man be washed in a sea or a pool, a stream or a fount, a lake or a trough; nor is there any distinction between those whom John baptized in the Jordan and those whom Peter baptized in the Tiber, unless withal the eunuch whom Philip baptized in the midst of his journeys with chance water, derived (therefrom) more or less of salvation than others. [Acts 8:26-40] All waters, therefore, in virtue of the pristine privilege of their origin, do, after invocation of God, attain the sacramental power of sanctification; for the Spirit immediately supervenes from the heavens, and rests over the waters, sanctifying them from Himself; and being thus sanctified, they imbibe at the same time the power of sanctifying. Albeit the similitude may be admitted to be suitable to the simple act; that, since we are defiled by sins, as it were by dirt, we should be washed from those stains in waters. But as sins do not show themselves in our flesh (inasmuch as no one carries on his skin the spot of idolatry, or fornication, or fraud), so persons of that kind are foul in the spirit, which is the author of the sin; for the spirit is lord, the flesh servant. Yet they each mutually share the guilt: the spirit, on the ground of command; the flesh, of subservience. Therefore, after the waters have been in a manner endued with medicinal virtue through the intervention of the angel, the spirit is corporeally washed in the waters, and the flesh is in the same spiritually cleansed.
Tertullian, On Baptism, Chapter 4