When two boats share the same hull type — in this case both the Seaway Boats 18 Sportsman 2013 and the Seaway Boats 21 Seafarer 2013 are deep vee designs with fiberglass construction — the buying decision usually comes down to a handful of practical questions: how many people are you putting on the water, how far do you trailer, and what does your tow vehicle weigh?
On paper these two are close siblings in the size department — Seaway Boats 18 Sportsman 2013 at 18,0 ft versus Seaway Boats 21 Seafarer 2013 at 21,0 ft. At 11 lbs and 23 lbs respectively, both sit in a similar weight class — either should pair comfortably with most mid-size SUVs and half-ton trucks, though always confirm your specific tow rating with the motor added.
The power gap is worth calling out. Rated to 90 hp, the Seaway Boats 21 Seafarer 2013 has a 40-hp advantage over the Seaway Boats 18 Sportsman 2013's 50-hp ceiling — enough to notice on acceleration and at cruising speed, particularly with a full passenger load.
For family outings this is probably the sharpest distinction between the two. The Seaway Boats 21 Seafarer 2013 is rated for 6 passengers, while the Seaway Boats 18 Sportsman 2013 caps at 5. If you're regularly pulling extended family or a group of friends onto the water, the extra seats on the Seaway Boats 21 Seafarer 2013 could be the deciding factor.
Bottom line: Choose the Seaway Boats 21 Seafarer 2013 if your priority is putting more people on the water — it handles 6 passengers and at 21,0 ft it has the deck room to back that rating up comfortably. The Seaway Boats 18 Sportsman 2013 is the smarter pick if you want a lighter, easier-to-trailer boat rated for 5 that costs less to run day-to-day.