In plain words
Just as waters enter the ever-full, unmoving ocean, so the one whom all desires enter attains peace—not one who longs for desires.
Word for word
As the waters enter the ocean that is being filled yet stands unmoving, so the one whom all desires enter attains peace, not the desirer of desires.
The commentators
Bhagavadgītābhāṣyam
विदुषः त्यक्तैषणस्य स्थितप्रज्ञस्य यतेरेव मोक्षप्राप्तिः, न तु असंन्यासिनः कामकामिनः इत्येतमर्थं दृष्टान्तेन प्रतिपादयिष्यन् आह —
आपूर्यमाणम् अद्भिः अचलप्रतिष्ठम् अचलतया प्रतिष्ठा अवस्थितिः यस्य तम् अचलप्रतिष्ठं समुद्रम् आपः सर्वतो गताः प्रविशन्ति स्वात्मस्थमविक्रियमेव सन्तं यद्वत् , तद्वत् कामाः विषयसंनिधावपि सर्वतः इच्छाविशेषाः यं पुरुषम् — समुद्रमिव आपः — अविकुर्वन्तः प्रविशन्ति सर्वे आत्मन्येव प्रलीयन्ते न स्वात्मवशं कुर्वन्ति, सः शान्तिं मोक्षम् आप्नोति, न इतरः कामकामी, काम्यन्त इति कामाः विषयाः तान् कामयितुं शीलं यस्य सः कामकामी, नैव प्राप्नोति इत्यर्थः ॥ ७० ॥
In plain words
Liberation belongs to the one who knows, who has let go of every craving, whose wisdom no longer wavers — to the renunciate, and to him alone. Not to the man who has renounced nothing and goes on wanting what he wants. To set this out with an example, the Lord says:
'Constantly filled by waters, yet with an unmoving foundation' — this describes the ocean: rivers pour into it from every direction, and yet the ocean itself stays exactly as it is, unchanged, resting in its own nature. In just the same way, whatever desires arrive — however many particular cravings show up, even when their objects are right there in front of him — they enter such a person the way rivers enter the ocean, without disturbing him at all. Every one of them simply dissolves into the Self, and none of them ever gets him under its control. That person attains peace, mokṣa. Not so for the other kind — the one who 'craves desired objects': desires simply mean the things that are craved, and someone whose very nature is to keep chasing after them never attains peace at all. That is the meaning.
Close to the Sanskrit
Of the knower, of one whose longings are abandoned, of one whose wisdom is steady, of the ascetic alone is the attainment of liberation — but not of the non-renunciate, of one who desires desires. Intending to establish this meaning by an illustration, he says —
'Being filled by waters' — 'having an unmoving foundation': that ocean whose foundation, whose steadiness, is unmoving — rivers, having gone from all sides, enter it, [the ocean] remaining situated in its own nature, unmodified. Just as [rivers enter the ocean], so too, in the same way, into whichever person desires — particular wishes — even in the presence of sense-objects, from all sides, enter, like waters into an ocean, without modifying him, and all dissolve in the Self alone, not bringing him under their control — that one attains 'peace,' liberation. Not the other, the 'one who craves desired objects' — 'desires' [means] objects, [called so because] they are desired; one whose habit is to crave those objects is 'one who craves desired objects' — he never attains it: such is the meaning. [End of commentary on verse 70.]