In a case like this, it is partly a matter of personal preference, and partly a matter of determining the meter used by the rest of the poem. To quote from Turco's The Book of Forms: A Handbook of Poetics (pp. 41-42):
Epiploce. This is a term used to describe a situation when, in scansion, a line of verse may be viewed in either of two ways, that is, as a catalectic trochaic tetrameter line (
/^|/^|/^|/), or as an acephaleous iambic tetrameter line (
/|^/|^/|^/). Generally, the matter can be settled by a scansion of the rest of the poem to determine the normative meter.
This rule of thumb easily settles the case for Kubla Khan: given that most of the poem is written in iambic meter, it makes sense that "Floated midway on the waves" should be scanned as "Float|ed mid|way on| the waves|;" to avoid conflicting with the rest of the poem.
At the same time, there isn't really a difference in how "Float|ed mid|way on| the waves|;" and "Floated| midway| on the| waves;" are pronounced, so it's also a matter of personal preference.
Hartman's Verse: An Introduction to Prosody (which is a fantastic read) explains the contradiction between catalectic trochaic tetrameter and acephaleous iambic tetrameter by citing Derek Attridge, a scholar who argues that there aren't feet in English poetry, but beats. The complexities of this argument are outside the scope of this answer, but anyone interested in learning more should take a look at Verse: An Introduction to Prosody, which provides a useful summary, or any one of Attridge's books on the subject.