He is saying that agriculture is the least capital intensive of the three. This is not about marginal product- the labor might be low productivity such that MPK is still low.
Saying agriculture isnt capital intensive is more or less axiomatic. Everyone starts with agriculture. In Smith's time what today uses tractors and machines was done with livestock. It blurs the distinction.
His trade comment is probably highly ambiguous. What types of trade? There are probably canal boats and barges that passively float along rivers and dont involve any direct labor at all. On the other hand goods could be moved by courier and be all labor.
The easiest way to measure labor productivity or labor/capital ratio for an occupation is wages.
It lists a wage of a textile worker as up to 16s and common laborers as 3s. Other laborers made within this range. London laborers made 20s but this seems to be the high end. Sailors made 15s.
It is also possible these are due to labor power. However, these are generally fairly open, nonunionized jobs so the main factor in wages would be productivity. This is stuff subject to child labor so guilds have limited power.
So, just based on these wage rates, and basic assumptions, it does look like sea trade is more capital intensive than most laboring jobs, but it depends on the job.